drm/imx: 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.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-7-alexandru-cosmin.gheorghe@arm.com