drm/sprd: check the platform_get_resource() return value

platform_get_resource() may fail and return NULL, so check it's value
before using it.

Reported-by: Zou Wei <zou_wei@huawei.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/20220117084156.9338-1-kevin3.tang@gmail.com

v1 -> v2:
- new patch