drm/i915/ddi: Add more sanity check to the encoder HW readout

drm/i915/ddi: Add more sanity check to the encoder HW readout

Check for reserved register field values and conflicting
transcoder->port mappings (both MST and non-MST mappings or multiple SST
mappings).

This is also needed for the next patch to determine if a port is in MST
mode during sanitization after HW readout.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107200836.10191-1-imre.deak@intel.com