Use a dummy xe_debugfs_register() if debugfs is not enabled and move all
debugfs-related files under `ifeq ($(CONFIG_DEBUG_FS),y)` in the
Makefile. This is similar to what was done for display in
commit 439987f6 ("drm/xe: don't build debugfs files when
CONFIG_DEBUG_FS=n").

This removes the following warning while loading xe with
CONFIG_DEUBG_FS=n:

	xe 0000:03:00.0: [drm] Create GT directory failed
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240808171121.2484237-1-lucas.demarchi@intel.comSigned-off-by: Lucas De Marchi <lucas.demarchi@intel.com>