drm/i915/icl: Find DSI presence for ICL

This patch detects DSI presence for ICL platform
by reading VBT. DSI detection is done while initializing
DSI using newly added function intel_gen11_dsi_init.

v2 by Jani:
 - Preserve old behavour of intel_bios_is_dsi_present()
 - s/intel_gen11_dsi_init/icl_dsi_init/g

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2324cdfc8918bda3165354e5e0d15053b1074f14.1540900289.git.jani.nikula@intel.com