drm/i915: Unconditionally clear plane visibility, v2.

We need to assume the plane has been visible before, even if no CRTC
is assigned to the plane. This is because when enabling a nv12 plane
on gen11, we will have to enable an extra plane and make it visible
by marking it in crtc_state->active_planes for
intel_update_planes_on_crtc().

Additionally, clear visible flag in intel_plane_atomic_check, in case
we ever hit a bug with visibility. Our code implicitly assumes that
plane_state->visible is only true when crtc and fb are set,
so we will either null deref in intel_fbc_choose_crtc() or
do something bad during the actual commit which cares even more.

Changes since v1:
- Unconditionally clear crtc_state->active_planes as well.
- Reword commit message, since this is now a preparation patch for
  NV12 Y / UV plane linking.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Clear nv12_planes in the beginning as well, clarify commit message]
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920102711.4184-4-maarten.lankhorst@linux.intel.com

drivers/gpu/drm/i915/intel_atomic_plane.c

@@ -117,24 +117,24 @@ int intel_plane_atomic_check_with_state(const struct intel_crtc_state *old_crtc_
 	struct intel_plane *intel_plane = to_intel_plane(plane);
 	int ret;
 
+	crtc_state->active_planes &= ~BIT(intel_plane->id);
+	crtc_state->nv12_planes &= ~BIT(intel_plane->id);
+	intel_state->base.visible = false;
+
+	/* If this is a cursor plane, no further checks are needed. */
 	if (!intel_state->base.crtc && !old_plane_state->base.crtc)
 		return 0;
 
-	intel_state->base.visible = false;
 	ret = intel_plane->check_plane(crtc_state, intel_state);
 	if (ret)
 		return ret;
 
 	/* FIXME pre-g4x don't work like this */
 	if (state->visible)
 		crtc_state->active_planes |= BIT(intel_plane->id);
-	else
-		crtc_state->active_planes &= ~BIT(intel_plane->id);
 
 	if (state->visible && state->fb->format->format == DRM_FORMAT_NV12)
 		crtc_state->nv12_planes |= BIT(intel_plane->id);
-	else
-		crtc_state->nv12_planes &= ~BIT(intel_plane->id);
 
 	return intel_plane_atomic_calc_changes(old_crtc_state,
 					       &crtc_state->base,
@@ -152,6 +152,7 @@ static int intel_plane_atomic_check(struct drm_plane *plane,
 	const struct drm_crtc_state *old_crtc_state;
 	struct drm_crtc_state *new_crtc_state;
 
+	new_plane_state->visible = false;
 	if (!crtc)
 		return 0;