drm/omap: Cast pointer to integer without generating warning

On 64-bit platforms, the compiler complains that casting a void pointer
to an unsigned int loses data. Cast the pointer to a uintptr_t to fix
this.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>