drm: Remove redundant NULL check

Fix below warnings reported by coccicheck:
./drivers/gpu/drm/drm_drv.c:819:2-7: WARNING: NULL check before some freeing functions is not needed.

Fixes: 5dad34f3 ("drm: Cleanups after drmm_add_final_kfree rollout")
Signed-off-by: Li Heng <liheng40@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1595474863-33112-1-git-send-email-liheng40@huawei.com