drm/xe/pmu: Extract xe_pmu_event_update()

Like other pmu drivers, keep the update separate from the read so it can
be called from other methods (like stop()) without side effects.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124050411.2189060-3-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>