drm/xe: Modify stepping info directly in xe_step_*_get()

drm/xe: Modify stepping info directly in xe_step_*_get()

In an upcoming change, we will add a member to struct xe_step_info to
represent the platform-level stepping.  As such, we should stop assigning
the value returned by functions xe_step_pre_gmdid_get() and
xe_step_gmdid_get() directly to xe->info.step.

Since there are no other users for those functions, let's simply update
them to modify xe->info.step directly.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patch.msgid.link/20260309-extra-nvl-p-enabling-patches-v5-1-be9c902ee34e@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>