drm/sprd: fix potential NULL dereference

'drm' could be null in sprd_drm_shutdown, and drm_warn maybe dereference
it, remove this warning log.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kevin Tang <kevin3.tang@gmail.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/all/20220117084044.9210-1-kevin3.tang@gmail.com

v1 -> v2:
- Split checking platform_get_resource() return value to a separate patch
- Use dev_warn() instead of removing the warning log