drm/panel: consolidate unnecessary explicit dependencies

DRM_PANEL_LD9040 and DRM_PANEL_S6E8AA0 both explicitly depended on
DRM_PANEL && DRM, whereas DRM_PANEL_SIMPLE relies upon the dependency
on the menu.

We do not need to use explicit dependencies if we make the menu depend
on DRM_PANEL && DRM - this will implicitly make each entry in the menu
depend on DRM_PANEL && DRM without this needing to be explicitly stated
against every entry.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>