drm: atmel-hlcdc: 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.

Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-5-alexandru-cosmin.gheorghe@arm.com

drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c

@@ -942,10 +942,7 @@ static void atmel_hlcdc_plane_reset(struct drm_plane *p)
 				"Failed to allocate initial plane state\n");
 			return;
 		}
-
-		p->state = &state->base;
-		p->state->alpha = DRM_BLEND_ALPHA_OPAQUE;
-		p->state->plane = p;
+		__drm_atomic_helper_plane_reset(p, &state->base);
 	}
 }