drm/i915/dsi: make the vbt panel driver use mipi_dsi_device for transfers

Use the drm core interfaces in preparation of removing our homebrew.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-By: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>