drm/i915/dsi: split out icl_dsi.h

Follow the convention of corresponding .h for .c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211122111504.223248-4-jani.nikula@intel.com