drm/i915: preserve pipe A quirk in i9xx_set_pipeconf

This regression has been introduced in

commit 9f11a9e4e50006b615ba94722dfc33ced89664cf
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jun 13 00:54:58 2013 +0200

    drm/i915: set up PIPECONF explicitly for i9xx/vlv platforms

Ville brough up the idea that this is just the pipe A quirk gone
wrong.

Note that after resume the bios might or might not have enabled pipe A
already.  We have a bit of magic to make sure that on resume we set up
a decent mode for pipe A, but I fear if I just smash pipe A to always
on we'd enable it in a bogus state and hang the hw. Hence the
readback.

v2: Clarify the logic a bit as suggested by Chris. Also amend the
commit message to clarify why we don't unconditionally enable the
pipe.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66462
References: https://lkml.org/lkml/2013/8/26/238
Cc: Meelis Roos <mroos@ut.ee>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Use |= instead of = as suggested by Chris.]
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -4775,6 +4775,10 @@ static void i9xx_set_pipeconf(struct intel_crtc *intel_crtc)
 
 	pipeconf = 0;
 
+	if (dev_priv->quirks & QUIRK_PIPEA_FORCE &&
+	    I915_READ(PIPECONF(intel_crtc->pipe)) & PIPECONF_ENABLE)
+		pipeconf |= PIPECONF_ENABLE;
+
 	if (intel_crtc->pipe == 0 && INTEL_INFO(dev)->gen < 4) {
 		/* Enable pixel doubling when the dot clock is > 90% of the (display)
 		 * core speed.