MAINTAINERS: Add Sam as reviewer for drm/panel

Sam has been helping out a lot with reviewing DRM panel patches. Add him
as reviewer to help him do this important work.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416075910.12015-1-thierry.reding@gmail.com

MAINTAINERS

@@ -5340,6 +5340,7 @@ T:	git git://anongit.freedesktop.org/drm/drm-misc
 
 DRM PANEL DRIVERS
 M:	Thierry Reding <thierry.reding@gmail.com>
+R:	Sam Ravnborg <sam@ravnborg.org>
 L:	dri-devel@lists.freedesktop.org
 T:	git git://anongit.freedesktop.org/drm/drm-misc
 S:	Maintained