drm/i915: move dmc to display.dmc

Move display dmc related members under drm_i915_private display
sub-struct.

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/7cb91222e099b96b82c74b5f086d50c43459d61b.1661346845.git.jani.nikula@intel.com