drm: rcar-du: Use __drm_atomic_helper_plane_reset instead of copying the logic

drm: rcar-du: 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.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-8-alexandru-cosmin.gheorghe@arm.com