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.

v2:
- New try with the NULL/"" param value issue resolved

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7c8223b68fdafbc72bee0bf5afdb2ab15261cf00.1713519628.git.jani.nikula@intel.com


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