drm/omap: dsi: Hack-fix DSI bus flags

Since commit b4935e3a ("drm/omap: Store bus flags in the
omap_dss_device structure") video mode flags are managed by the omapdss
(and later omapdrm) core based on bus flags stored in omap_dss_device.
This works fine for all devices whose video modes are set by the omapdss
and omapdrm core, but breaks DSI operation as the DSI still uses legacy
code paths and sets the DISPC timings manually.

To fix the problem properly we should move the DSI encoder to the new
encoder model. This will however require a considerable amount of work.
Restore DSI operation by adding back video mode flags handling in the
DSI encoder driver as a hack in the meantime.

Fixes: b4935e3a ("drm/omap: Store bus flags in the omap_dss_device structure")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111035120.20668-5-laurent.pinchart@ideasonboard.com