drm/virtio: Replace instances of reference/unreference with get/put

Replace reference/unreference with get/put as it is consistent
with the kernel coding style. Done using the following semantic
patch by coccinelle.

@r@
expression e;
@@

-drm_gem_object_unreference_unlocked(e);
+drm_gem_object_put_unlocked(e);

Signed-off-by: Srishti Sharma <srishtishar@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1506679419-7130-1-git-send-email-srishtishar@gmail.com