drm/sun4i: backend: Support alpha property on lowest plane

Unlike what we previously thought, only the per-pixel alpha is broken on
the lowest plane and the per-plane alpha isn't. Remove the check on the
alpha property being set on the lowest plane to reject a mode.

Fixes: dcf496a ("drm/sun4i: sun4i: Introduce a quirk for lowest plane alpha support")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728134810.883457-1-maxime@cerno.tech

Author: mripard

drivers/gpu/drm/sun4i/sun4i_backend.c

@@ -589,8 +589,7 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
 
 	/* We can't have an alpha plane at the lowest position */
 	if (!backend->quirks->supports_lowest_plane_alpha &&
-	    (plane_states[0]->fb->format->has_alpha ||
-	    (plane_states[0]->alpha != DRM_BLEND_ALPHA_OPAQUE)))
+	    (plane_states[0]->alpha != DRM_BLEND_ALPHA_OPAQUE))
 		return -EINVAL;
 
 	for (i = 1; i < num_planes; i++) {