drm/i915/dsi: move connector mode functions to common file

Move DSI connector functions to intel_dsi.c and make them available to
both legacy and ICL DSI.

v2 by Jani:
 - Move the functions to intel_dsi.c
 - Don't reuse intel_dsi_connector_destroy()

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/99244c6edf4a26ef2e279c7160d22dbbb5cd95f2.1540900289.git.jani.nikula@intel.com