drm/amd/display: Use drm helper for resetting plane state

[Why]
To help prevent plane state not being set to the correct default
value if any new properties are added in the future.

[How]
Use the drm helper - which seems to be the common solution among other
DRM drivers.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -3657,13 +3657,8 @@ static void dm_drm_plane_reset(struct drm_plane *plane)
 	amdgpu_state = kzalloc(sizeof(*amdgpu_state), GFP_KERNEL);
 	WARN_ON(amdgpu_state == NULL);
 
-	if (amdgpu_state) {
-		plane->state = &amdgpu_state->base;
-		plane->state->plane = plane;
-		plane->state->rotation = DRM_MODE_ROTATE_0;
-		plane->state->alpha = DRM_BLEND_ALPHA_OPAQUE;
-		plane->state->pixel_blend_mode = DRM_MODE_BLEND_PREMULTI;
-	}
+	if (amdgpu_state)
+		__drm_atomic_helper_plane_reset(plane, &amdgpu_state->base);
 }
 
 static struct drm_plane_state *