MAINTAINERS: drm/panel is now maintained in drm-misc

drm/panel is a good candidate for the drm-misc group maintainership and
it's been maintained in the common drm-misc tree for a release, so make
it official.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170814140011.1336-1-thierry.reding@gmail.com