drm/i915/mso: add splitter state readout for platforms that support it

Add splitter configuration to crtc state, and read it where
supported. Also add splitter state dumping. The stream splitter will be
required for eDP MSO.

v4:
- Catch invalid splitter configuration (Uma)

v3:
- Convert segment timings to full panel timings.
- Refer to splitter instead of mso in crtc state.
- Dump splitter state.

v2: Add warning for mso being enabled on pipes other than A.

Cc: Nischal Varide <nischal.varide@intel.com>
Cc: Uma Shankar <uma.shankar@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/95cbe1c9d45edf3e3ec252e49fb49055def98155.1614682842.git.jani.nikula@intel.com