drm/xe: Assert size of the struct xe_reg

We want to keep the struct xe_reg as small as possible.
Make sure we don't accidentally change its size.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240314173130.1177-2-michal.wajdeczko@intel.com


Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>