drm/i915/snps: constify struct intel_mpllb_state arrays harder

The tables should be const arrays of const pointers, not just arrays of
const pointers.

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210825145811.4227-1-jani.nikula@intel.com