drm/i915/display: move dmc_firmware_path to display params

The dmc_firmware_path parameter is clearly a display parameter. Move it
there so it's available to both i915 and xe modules. This also cleans up
the ugly member in struct xe_device.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240321161856.3517856-1-jani.nikula@intel.com


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