drm/i915/dmc: add for_each_dmc_id() and use it

The loop is duplicated many times, with slightly different ways. Unify.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8dcaa716093e6fbe75bb69ee7ac715a3f007a523.1675339447.git.jani.nikula@intel.com