drm/i915/mtl: Initialize empty clockgating hooks for MTL

Clock gating hooks to be initialized for MTL are yet to be implemented.
Use a nop till we identify relevant WA's here.

Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111235531.3353815-3-radhakrishna.sripada@intel.com