drm/vkms: Remove set but not used variable 'vkms_obj'

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

drivers/gpu/drm/vkms/vkms_plane.c: In function 'vkms_prepare_fb':
drivers/gpu/drm/vkms/vkms_plane.c:144:26: warning:
 variable 'vkms_obj' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit
8ce1bb0b ("drm/vkms: map/unmap buffers in [prepare/cleanup]_fb hooks")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1543634444-186448-1-git-send-email-yuehaibing@huawei.com

drivers/gpu/drm/vkms/vkms_plane.c

@@ -141,14 +141,12 @@ static int vkms_prepare_fb(struct drm_plane *plane,
 			   struct drm_plane_state *state)
 {
 	struct drm_gem_object *gem_obj;
-	struct vkms_gem_object *vkms_obj;
 	int ret;
 
 	if (!state->fb)
 		return 0;
 
 	gem_obj = drm_gem_fb_get_obj(state->fb, 0);
-	vkms_obj = drm_gem_to_vkms_gem(gem_obj);
 	ret = vkms_gem_vmap(gem_obj);
 	if (ret)
 		DRM_ERROR("vmap failed: %d\n", ret);