drm: omapdrm: Fix plane state free in plane reset handler

The plane reset handler frees the plane state and allocates a new
default state, but when doing so attempt to free the plane state using
the base plane state pointer instead of casting it to the
driver-specific state object that has been allocated. Fix it by using
the omap_plane_atomic_destroy_state() function to destroy the plane
state instead of duplicating the code.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[tomi.valkeinen@ti.com: move of the func into separate patch]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>