drm/xe: Handle polling only for system s/r in xe_display_pm_suspend/resume()

This is a preparation for the follow-up patch where polling
will be handled properly for all cases during runtime suspend/resume.

v2: rebased

Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240823112148.327015-3-vinod.govindapillai@intel.com