drm/i915/guc: remove writes to GEN8_DRBREG registers

These registers are not actually writable by the CPU; only the GuC can
actually program them. So let's not do writes that have no effect.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>