drm/i915/dsi: move operation mode types to intel_dsi.h

Reduce clutter from intel_drv.h with the minimal change.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/72de677e844220d8522a836aae206c278ea45284.1556809195.git.jani.nikula@intel.com