drm/i915/gvt: fix dropping obj reference twice

The reference count of obj will be decremented twice if error occurs
in dma_buf_fd(). Additionally, attempting to read the reference count of
obj after dropping reference may lead to a use after free bug. Here, we
drop obj's reference until it is not used.

Fixes: e546e281 ("drm/i915/gvt: Dmabuf support for GVT-g")
Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>