drm/i915/reg: add stream splitter configuration definitions

The splitter configuration is required for eDP MSO.

Bspec: 50174
Cc: Nischal Varide <nischal.varide@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/443ad1fbf908800ee4e09315cb6a7ba26c64d136.1613054234.git.jani.nikula@intel.com