drm: prime: fix refcounting on the dmabuf import error path

In commit be8a42ae60 we inroduced a refcount problem, where on the
drm_gem_prime_fd_to_handle() error path we'll call dma_buf_put() for
self imported dma buffers.

Fix this by taking a reference on the dma buffer in the .gem_import
hook instead of assuming the caller had taken one. Besides fixing the
bug this is also more logical.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>