drm/i915/dmc: split out dmc registers to a separate file

Clean up the massive i915_reg.h a bit with this isolated set of
registers.

v2: Remove stale comment (Lucas)

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/20220330113417.220964-3-jani.nikula@intel.com