drm/vmwgfx: remove unused vmw_bo_is_vmw_bo() declaration

vmw_bo_is_vmw_bo() has been removed since
commit 298799a2 ("drm/vmwgfx: Fix gem refcounting and
memory evictions"), so remove it.
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913024847.552254-2-cuigaosheng1@huawei.com

drivers/gpu/drm/vmwgfx/vmwgfx_drv.h

@@ -877,7 +877,6 @@ static inline void vmw_user_resource_noref_release(void)
 /**
  * Buffer object helper functions - vmwgfx_bo.c
  */
-extern bool vmw_bo_is_vmw_bo(struct ttm_buffer_object *bo);
 extern int vmw_bo_pin_in_placement(struct vmw_private *vmw_priv,
 				   struct vmw_buffer_object *bo,
 				   struct ttm_placement *placement,