drm/i915/dsi: stop using the drm_panel framework completely

Now that we've stopped using the drm_panel hooks, there aren't any
benefits left with using the drm_panel framework. Remove the rest of the
drm_panel use. No functional changes.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/6602e36641451952065092401bd6e6cfbe93e208.1488810382.git.jani.nikula@intel.com