Move most of the SKL+ universal plane register definitions
into their own file. Declutters i915_reg.h a bit more.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
CC: Zhi Wang <zhi.wang.linux@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510152329.24098-3-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>