drm: virtio: replace reference/unreference with get/put

DRM core uses reference/unreference suffixes for refcounting
functions, but kernel uses get/put (e.g. kref_get/put()).
Replace reference/unreference with get/put for consistency
and also it's shorter.

The following cocci script was used to generate the patch:

@@
expression e;
@@

(
-drm_gem_object_reference(e);
+drm_gem_object_get(e);
|
-drm_gem_object_unreference(e);
+drm_gem_object_put(e);
|
-drm_gem_object_unreference_unlocked(e);
+drm_gem_object_put_unlocked(e);
)

Signed-off-by: Aastha Gupta <aastha.gupta4104@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1508776686-29664-1-git-send-email-aastha.gupta4104@gmail.com