drm/msm: remove redundant NULL check

Fix below warnings reported by coccicheck:
./drivers/gpu/drm/msm/msm_gem.c:991:3-9: WARNING: NULL check before some
freeing functions is not needed.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Zhong <abaci-bugfix@linux.alibaba.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/msm_gem.c

@@ -987,8 +987,7 @@ void msm_gem_free_object(struct drm_gem_object *obj)
 		/* Don't drop the pages for imported dmabuf, as they are not
 		 * ours, just free the array we allocated:
 		 */
-		if (msm_obj->pages)
-			kvfree(msm_obj->pages);
+		kvfree(msm_obj->pages);
 
 		put_iova_vmas(obj);