drm/sprd: fix potential NULL dereference

In the Linux kernel, the following vulnerability has been resolved: drm/sprd: fix potential NULL dereference 'drm' could be null in sprd_drm_shutdown, and drm_warn maybe dereference it, remove this warning log. v1 -> v2: - Split checking platform_get_resource() return value to a separate patch - Use dev_warn() instead of removing the warning log