drm/i915/gt: Fix platform prefix

Different handling for XeHP and later platforms should be using the
xehp prefix, not gen125. Rename them.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220930050903.3479619-4-lucas.demarchi@intel.com