drm/panel: simple: Add display timing support

The simple panel driver's ->get_modes() implementation calculates the
display mode list from the typical timings and the ->get_timings()
implementation returns the timings to the connected encoder for mode
validation and fixup.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
[treding@nvidia.com: select VIDEOMODE_HELPERS]
Signed-off-by: Thierry Reding <treding@nvidia.com>