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
8ce1bb0b5337 ("drm/vkms: map/unmap buffers in [prepare/cleanup]_fb hooks")

Signed-off-by: YueHaibing <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vkms/vkms_plane.c

···
 			   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);