video: Kconfig: move drm and fb into separate menus

At the moment the "Device Drivers / Graphics support" kernel config page
looks rather messy, with DRM and fbdev driver selections on the same
page, some on the top level Graphics support page, some under their
respective subsystems.

If I'm not mistaken, this is caused by the drivers depending on other
things than DRM or FB, which causes Kconfig to arrange the options in
not-so-neat manner.

Both DRM and FB have a main menuconfig option for the whole DRM or FB
subsystem. Optimally, this would be enough to arrange all DRM and FB
options under the respective subsystem, but for whatever reason this
doesn't work reliably.

This patch adds an explicit submenu for DRM and FB, making it much
clearer which options are related to FB, and which to DRM.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>