drm/i915: Disable FBC on Ironlake to save 1W

Frame buffer compression is broken on Ironlake due to buggy hardware.
Currently it is disabled through chicken bits, but it still consumes
over 1W more than if we simply never attempt to enable the FBC code
paths.

Signed-off-by: Alex Shi <alex.shi@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org

drivers/gpu/drm/i915/i915_drv.c

@@ -150,7 +150,8 @@ static const struct intel_device_info intel_ironlake_d_info = {
 
 static const struct intel_device_info intel_ironlake_m_info = {
 	.gen = 5, .is_mobile = 1,
-	.need_gfx_hws = 1, .has_fbc = 1, .has_rc6 = 1, .has_hotplug = 1,
+	.need_gfx_hws = 1, .has_rc6 = 1, .has_hotplug = 1,
+	.has_fbc = 0, /* disabled due to buggy hardware */
 	.has_bsd_ring = 1,
 };