drm/xe: Update runtime detection of has_flat_ccs

drm/xe: Update runtime detection of has_flat_ccs

It's confusing to have a *set* function that actually probes the
hardware rather than receiving a parameter. Rename it to *probe* along
with prefix removal and comment in the relevant places that the
has_flat_ccs flag may be overridden in runtime.

While at it, fix the mixed declaration of struct xe_gt.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240904162238.2831202-2-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>