drm/i915/icl: Make common DSI functions available

This patch moves couple of legacy DSI functions to header and common DSI
files so that they can be re-used by Gen11 DSI. No functional change.

v2 by Jani:
 - Move intel_dsi_msleep() to intel_dsi_vbt.c

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/bd1f5f3e96d3e1de4d359f4fd1b750ac7e3c87d4.1539613303.git.jani.nikula@intel.com