drm/i915/mtl: Fix rawclk for Meteorlake PCH

MTL has a fixed rawclk of 38400Khz. Register does not need to be
reprogrammed.

Bspec: 49304

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220818234202.451742-13-radhakrishna.sripada@intel.com