drm/i915/wm: split out SKL+ watermark regs to a separate file

Clean up i915_reg.h by splitting out SKL+ watermark regs to
display/skl_watermark_regs.h.

v2: Rebased

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com> # v1
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230331090949.2858951-1-jani.nikula@intel.com