drm: Add support for extracting sync signal drive edge from videomode

The sync in some panels needs to be driven by different edge of the pixel
clock compared to data. This is reflected by the
DISPLAY_FLAGS_SYNC_(POS|NEG)EDGE in videmode flags.
Add similar similar definitions for bus_flags and convert the sync drive
edge via drm_bus_flags_from_videomode().

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180618132242.8673-2-tomi.valkeinen@ti.com