drm/i915/dmc: move dmc debugfs to intel_dmc.c

Continue localizing DMC register and data access to intel_dmc.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3b7bc0fc117b030d59dd74891af104c165bf9b39.1647870374.git.jani.nikula@intel.com