drm/xe: Return error if drm_buddy_init() fails

Fix warning:
	../drivers/gpu/drm/xe/xe_ttm_vram_mgr.c: In function ‘__xe_ttm_vram_mgr_init’:
	../drivers/gpu/drm/xe/xe_ttm_vram_mgr.c:340:13: error: variable ‘err’ set but not used [-Werror=unused-but-set-variable]
	  340 |         int err;
	      |             ^~~

Check for the error return and return it, like done by other drivers.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>