drm/virtio: remove set but not used variable 'vgdev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/virtio/virtgpu_ttm.c: In function 'virtio_gpu_init_mem_type':
drivers/gpu/drm/virtio/virtgpu_ttm.c:117:28: warning:
 variable 'vgdev' set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/virtio/virtgpu_ttm.c: In function 'virtio_gpu_bo_swap_notify':
drivers/gpu/drm/virtio/virtgpu_ttm.c:300:28: warning:
 variable 'vgdev' set but not used [-Wunused-but-set-variable]

It is never used since introduction in dc5698e8

 ("Add virtio gpu driver.")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190325092631.152060-1-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>