drm/panel: Add display timing support

Many panel data sheets, additionally to typical values, list allowed
ranges for timings such as hsync/vsync lengths, porches, and the pixel
clock rate. These can be stored in a struct display_timing, to be used
by an encoder mode_fixup callback to clamp user provided timing values
or to validate workarounds for clock source limitations.

This patch adds a new drm_panel_funcs callback that returns the panel's
available display_timing entries.

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