drm/xe: make GT sysfs init return void

Currently return from xe_gt_sysfs_init() is ignored
and also a failure in xe_gt_sysfs_init() isn't fatal
so make it return void.

V2 :
   - add drm_warn in error paths - Himal
   - Edit commit message - Nirmoy

Acked-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>