drm: omapdrm: Take GEM object reference when exporting dmabuf

To ensure that neither the GEM object nor the DRM device goes away while
a GEM object exported through dma-buf is still accessible, references
must be taken to both the GEM object and the DRM device at export time.
The dma-buf release handler already releases the GEM object, but the
export handler doesn't take a corresponding reference, which results in
a refcount underflow.

Fix this by replacing the custom implementation by
drm_gem_dmabuf_export() and drm_gem_dmabuf_release() that handle
reference counting for us.

Fixes: 6ad11bc3 ("staging: drm/omap: dmabuf/prime support")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>