drm/i915: Switch DDC when reading the EDID

The pre-retina MacBook Pro uses an LVDS panel and a gmux controller
to switch the panel between its two GPUs. The panel mode in VBIOS
is notoriously bogus on these machines and some models have no
VBIOS at all.

Use drm_get_edid_switcheroo() in lieu of drm_get_edid() on LVDS
if the vga_switcheroo handler is capable of temporarily switching
the panel's DDC lines to the integrated GPU. This allows us to
retrieve the EDID if the panel is currently muxed to the discrete GPU.

This only enables EDID probing on the pre-retina MBP (2008 - 2013).
The retina MBP (2012 - present) uses eDP and gmux is not capable of
switching AUX separately from the main link on these models.
This will be addressed in later patches.

List of pre-retina MBPs with dual GPUs, one of them Intel:
    [MBP  6,2 2010  intel ILK + nvidia GT216  pre-retina  15"]
    [MBP  6,1 2010  intel ILK + nvidia GT216  pre-retina  17"]
    [MBP  8,2 2011  intel SNB + amd turks     pre-retina  15"]
    [MBP  8,3 2011  intel SNB + amd turks     pre-retina  17"]
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]

v3: Commit newly added due to introduction of drm_get_edid_switcheroo()
    wrapper which drivers need to opt-in to.

v5: Rebase on "vga_switcheroo: Add handler flags infrastructure",
    i.e. call drm_get_edid_switcheroo() only if the handler
    indicates that DDC is switchable.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115
Tested-by: Lukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/bb095e14a2259be7fdd10092f9d6874a9be8f27b.1452525860.git.lukas@wunner.de

drivers/gpu/drm/i915/intel_lvds.c

@@ -31,6 +31,7 @@
 #include <linux/dmi.h>
 #include <linux/i2c.h>
 #include <linux/slab.h>
+#include <linux/vga_switcheroo.h>
 #include <drm/drmP.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_crtc.h>
@@ -1080,7 +1081,12 @@ void intel_lvds_init(struct drm_device *dev)
 	 * preferred mode is the right one.
 	 */
 	mutex_lock(&dev->mode_config.mutex);
-	edid = drm_get_edid(connector, intel_gmbus_get_adapter(dev_priv, pin));
+	if (vga_switcheroo_handler_flags() & VGA_SWITCHEROO_CAN_SWITCH_DDC)
+		edid = drm_get_edid_switcheroo(connector,
+				    intel_gmbus_get_adapter(dev_priv, pin));
+	else
+		edid = drm_get_edid(connector,
+				    intel_gmbus_get_adapter(dev_priv, pin));
 	if (edid) {
 		if (drm_add_edid_modes(connector, edid)) {
 			drm_mode_connector_update_edid_property(connector,