drm/virtio: Simplify virtio_gpu_primary_plane_update workflow.

Return early for the no framebuffer (or disabled output) case.
Results in a simpler code flow for the remaining cases.
No functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20191023062539.11728-3-kraxel@redhat.com