drm/i915: Force a LUT update in intel_initial_commit()

If we force a plane update to fix up our half populated plane state
we'll also force on the pipe gamma for the plane (since we always
enable pipe gamma currently). If the BIOS hasn't programmed a sensible
LUT into the hardware this will cause the image to become corrupted.
Typical symptoms are a purple/yellow/etc. flash when the driver loads.

To avoid this let's program something sensible into the LUT when
we do the plane update. In the future I plan to add proper plane
gamma enable readout so this is just a temporary measure.

Cc: Hans de Goede <hdegoede@redhat.com>
Fixes: 516a49cc1946 ("drm/i915: Fix assert_plane() warning on bootup with external display")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181120135450.3634-1-ville.syrjala@linux.intel.com
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
(cherry picked from commit fa6af5145b4e87a30a530be0d80734a9dd40da77)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -15267,6 +15267,14 @@ retry:
 			ret = drm_atomic_add_affected_planes(state, crtc);
 			if (ret)
 				goto out;
+
+			/*
+			 * FIXME hack to force a LUT update to avoid the
+			 * plane update forcing the pipe gamma on without
+			 * having a proper LUT loaded. Remove once we
+			 * have readout for pipe gamma enable.
+			 */
+			crtc_state->color_mgmt_changed = true;
 		}
 	}