drm/i915/dmc: split out per-platform firmware path selection

The big if ladder clutters intel_dmc_init(). Split it out to a separate
function.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fe3d7b2b18c317a2f924f2023271d38afee3b18a.1713519628.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>