drm/xe: Fix missing runtime PM reference in ccs_mode_store

In the Linux kernel, the following vulnerability has been resolved:

drm/xe: Fix missing runtime PM reference in ccs_mode_store

ccs_mode_store() calls xe_gt_reset() which internally invokes
xe_pm_runtime_get_noresume(). That function requires the caller
to already hold an outer runtime PM reference and warns if none
is held:

[46.891177] xe 0000:03:00.0: [drm] Missing outer runtime PM protection
[46.891178] WARNING: drivers/gpu/drm/xe/xe_pm.c:885 at
xe_pm_runtime_get_noresume+0x8b/0xc0

Fix this by protecting xe_gt_reset() with the scope-based
guard(xe_pm_runtime)(xe), which is the preferred form when
the reference lifetime matches a single scope.

v2:
- Use scope-based guard(xe_pm_runtime)(xe) (Shuicheng)
- Update commit message accordingly

(cherry picked from commit 7937ea733f79b3f25e802a0c8360bf7423856f36)