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

Add a name to the dmc id validity check.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/311213ce328575a95d793a219d4dac7d947086cc.1675339447.git.jani.nikula@intel.com