drm/xe: Convert gsc_work from mem_access to xe_pm_runtime

Let's directly use xe_pm_runtime_{get,put} instead of the
mem_access helpers that are going away soon.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222163937.138342-11-rodrigo.vivi@intel.com