videomode: simplify videomode Kconfig and Makefile

videomode: simplify videomode Kconfig and Makefile

This patch simplifies videomode related Kconfig and Makefile. After this
patch, there's only one non-user selectable Kconfig option left,
VIDEOMODE_HELPERS. The reasons for the change:

* Videomode helper functions are not something that should be shown in
  the kernel configuration options. The related code should just be
  included if it's needed, i.e. selected by drivers using videomode.

* There's no need to have separate Kconfig options for videomode and
  display_timing. First of all, the amount of code for both is quite
  small. Second, videomode depends on display_timing, and display_timing
  in itself is not really useful, so both would be included in any case.

* CONFIG_VIDEOMODE is a bit vague name, and CONFIG_VIDEOMODE_HELPERS
  describes better what's included.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>