Revert "drm/i915/display: move dmc_firmware_path to display params"

This reverts commit 0d82a0d6.

The commit failed to take into account that the parameter duplication
and debugfs for char * parameters were subtly different between i915
core and display, and caused the DMC and PM to be disabled. Moreover,
the patch was pushed with failing CI results. Revert, and get back to
the drawing board.

Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>