drm/i915/dsi: fix pipe D readout for DSI transcoders

Commit 4d89adc7 ("drm/i915/display/dsi: Add support to pipe D")
added pipe D support for DSI, but failed to update the state readout.

Fixes: 4d89adc7 ("drm/i915/display/dsi: Add support to pipe D")
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191211110844.2996-1-jani.nikula@intel.com