drm/i915: Generalize the async flip capability check

Only assign the plane->async_flip() vfunc when the plane supports
async flips. For now we keep this artificially limited to the primary
plane since thats the only thing the legacy page flip uapi can target
and there is no async flip support in the atomic uapi yet.

Cc: Karthik B S <karthik.b.s@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111163711.12913-5-ville.syrjala@linux.intel.comReviewed-by: Karthik B S <karthik.b.s@intel.com>

drivers/gpu/drm/i915/display/intel_display.c

@@ -12254,7 +12254,7 @@ static int intel_atomic_check_async(struct intel_atomic_state *state)
 		 * this(vlv/chv and icl+) should be added when async flip is
 		 * enabled in the atomic IOCTL path.
 		 */
-		if (plane->id != PLANE_PRIMARY)
+		if (!plane->async_flip)
 			return -EINVAL;
 
 		/*

drivers/gpu/drm/i915/display/intel_sprite.c

@@ -3290,7 +3290,9 @@ skl_universal_plane_create(struct drm_i915_private *dev_priv,
 	plane->get_hw_state = skl_plane_get_hw_state;
 	plane->check_plane = skl_plane_check;
 	plane->min_cdclk = skl_plane_min_cdclk;
-	plane->async_flip = skl_plane_async_flip;
+
+	if (plane_id == PLANE_PRIMARY)
+		plane->async_flip = skl_plane_async_flip;
 
 	if (INTEL_GEN(dev_priv) >= 11)
 		formats = icl_get_plane_formats(dev_priv, pipe,