drm/i915/ddi: split up intel_ddi_read_func_ctl() by output type
The function has become quite long, and the switch-case statement quite
complex with the fallthrougs. Simplify by splitting to individual
functions and an if-ladder.
This highlights how TRANS_DDI_MODE_SELECT_FDI_OR_128B132B works on
different platforms.
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2621df6e6b0b7ac75159cfb112755c35b30ce906.1732106557.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>