drm/i915: Adjust DSI fixed mode handling

DSI has its own convoluted way of grabbing the fixed mode from
the VBT. Change it to follow the path laid out by LVDS/eDP.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321132446.22394-4-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jani Nikula <jani.nikula@intel.com>