drm: mali-dp: 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.

Reviewed-by: Ayan Kumar halder <ayan.halder@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-4-alexandru-cosmin.gheorghe@arm.com

drivers/gpu/drm/arm/malidp_planes.c

@@ -78,11 +78,8 @@ static void malidp_plane_reset(struct drm_plane *plane)
 	kfree(state);
 	plane->state = NULL;
 	state = kzalloc(sizeof(*state), GFP_KERNEL);
-	if (state) {
-		state->base.plane = plane;
-		state->base.rotation = DRM_MODE_ROTATE_0;
-		plane->state = &state->base;
-	}
+	if (state)
+		__drm_atomic_helper_plane_reset(plane, &state->base);
 }
 
 static struct