drm/i915: Allow the initial async flip to change modifier

With Xorg+modesetting on skl+ we see the following behaviour:
1. root pixmap is X-tiled
2. client submitted buffers can be Y-tiled (w/ 'Option "dmabuf_capable"')
3. we try to switch from the X-tiled buffer to the Y-tiled buffer
   using an async flip (when vsync is disabled).
4. the async flip will be rejected by i915 due to the modifier change

Relax the rules a bit by turning the first async flip into a sync
flip so that we can change the modifier if necessary. Note that
we already convert the first async flip into a sync flip on adl+
in order to reprogram the watermarks.
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240430095639.26390-4-ville.syrjala@linux.intel.com

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

@@ -429,10 +429,20 @@ static bool intel_plane_do_async_flip(struct intel_plane *plane,
 	 * In platforms after DISPLAY13, we might need to override
 	 * first async flip in order to change watermark levels
 	 * as part of optimization.
-	 * So for those, we are checking if this is a first async flip.
-	 * For platforms earlier than DISPLAY13 we always do async flip.
+	 *
+	 * And let's do this for all skl+ so that we can eg. change the
+	 * modifier as well.
+	 *
+	 * TODO: For older platforms there is less reason to do this as
+	 * only X-tile is supported with async flips, though we could
+	 * extend this so other scanout parameters (stride/etc) could
+	 * be changed as well...
+	 *
+	 * FIXME: Platforms with need_async_flip_disable_wa==true will
+	 * now end up doing two sync flips initially. Would be nice to
+	 * combine those into just the one sync flip...
 	 */
-	return DISPLAY_VER(i915) < 13 || old_crtc_state->uapi.async_flip;
+	return DISPLAY_VER(i915) < 9 || old_crtc_state->uapi.async_flip;
 }
 
 static bool i9xx_must_disable_cxsr(const struct intel_crtc_state *new_crtc_state,

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

@@ -6166,6 +6166,13 @@ static int intel_async_flip_check_hw(struct intel_atomic_state *state, struct in
 			return -EINVAL;
 		}
 
+		/*
+		 * We turn the first async flip request into a sync flip
+		 * so that we can reconfigure the plane (eg. change modifier).
+		 */
+		if (!new_crtc_state->do_async_flip)
+			continue;
+
 		if (old_plane_state->view.color_plane[0].mapping_stride !=
 		    new_plane_state->view.color_plane[0].mapping_stride) {
 			drm_dbg_kms(&i915->drm,