drm/omap: Use drm_gem_mmap_obj() to implement dma-buf mmap

The dma-buf mmap code was copied from the GEM mmap implementation.
Replace it with the new drm_gem_mmap_obj() function.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>