drm/virtio: Factor out common dmabuf unmapping code

Move out dmabuf detachment and unmapping into separate function. This
removes duplicated code and there is no need to check the GEM's kref now,
since both bo->attached and bo->sgt are unset under held reservation lock.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241202053955.2451321-1-dmitry.osipenko@collabora.com