drm/i915/dsi: switch to drm_panel interface

Replace intel_dsi_device and intel_dsi_dev_ops with drm_panel and
drm_panel_funcs. They are adequate for what we have now, and if we end
up needing more than this we should improve drm_panel. This will keep us
better aligned with the drm core infrastructure.

The panel driver initialization changes a bit. It still remains hideous,
but fixing that is beyond the scope here.

v2: extend mode config mutex to cover drm_panel_get_modes (Shobhit)
    vbt_panel->intel_dsi = intel_dsi in vbt panel init (Shobhit)

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>