drm/i915/dmc: use has_dmc_id_fw() instead of poking dmc->dmc_info directly

This will help in follow-up changes.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301122944.1298929-2-jani.nikula@intel.com