drm/debugfs: disallow debugfs access when device isn't registered

During device bringup it might be that we can't access the debugfs files.
Return -ENODEV until the registration is completed on access.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230829110115.3442-3-christian.koenig@amd.com