drm/vc4: Use __drm_atomic_helper_plane_reset instead of copying the logic

A new helper function(__drm_atomic_helper_plane_reset) has been added
for linking a plane with its state and resetting the core
properties(alpha, rotation, etc.) to their default values.
Use that instead of duplicating the logic.

__drm_atomic_helper_plane_reset initializes the alpha property to its
max value, which is defined by the drm core as DRM_BLEND_ALPHA_OPAQUE,
so nothing changes regarding the alpha value.
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Acked-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-10-alexandru-cosmin.gheorghe@arm.com

drivers/gpu/drm/vc4/vc4_plane.c

@@ -200,9 +200,7 @@ static void vc4_plane_reset(struct drm_plane *plane)
 	if (!vc4_state)
 		return;
 
-	plane->state = &vc4_state->base;
-	plane->state->alpha = DRM_BLEND_ALPHA_OPAQUE;
-	vc4_state->base.plane = plane;
+	__drm_atomic_helper_plane_reset(plane, &vc4_state->base);
 }
 
 static void vc4_dlist_write(struct vc4_plane_state *vc4_state, u32 val)