drm/armada: use drm_plane_helper_check_state()

Use drm_plane_helper_check_state() to check the overlay plane state
rather than drm_plane_helper_check_update(), as:

(1) using drm_plane_helper_check_state() provides a better migration
    path to atomic modeset
(2) it avoids needless copies of drm rectangle structures, and so is
    more efficient.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>