staging: drm/omap: dmabuf/prime support

For now just implementing the exporting APIs, not yet importing.  And
kmap is rejected on tiled buffers (although the usefulness of that seems
questionable, but could be added later if needed).

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>