drm/i915/dmc: add proper name to dmc id enum and use it

Clarify DMC ID usage by adding enum intel_dmc_id name to the enum, and
use dmc_id as the variable name for it throughout.

Convert a switch-case to if-ladder to avoid warnings about not handling
DMC_FW_MAX enumeration constant in the switch-case.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6912ccb411bb957c68c108b774745dbc7e0cbdc2.1675339447.git.jani.nikula@intel.com