drm/xe/reg_sr: Stop setting all whitelist slots

Currently xe_reg_sr_apply_whitelist() sets the unused values to a known
used value for no good reason: it could just leave it with the HW
default. The behavior is slightly different if there are no whitelist
registers for the engine as the function returns early. This is not
needed, so just drop the addition writes for the unused slots.

Later this will allow to reduce the amount of registers passed to GuC
for save/restore.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241209232739.147417-5-lucas.demarchi@intel.com


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