drm/i915/icl: Move dsi host init code to common file

This patch moves intl_dsi_host_init() code to intel_dsi.c so that legacy
and gen11 DSI code can share this code.

v2 by Jani:
 - Move the shared stuff to intel_dsi.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/1ee42b2d3c639e3f3c14a2c1595b8778901574d4.1540900289.git.jani.nikula@intel.com