drm/omap: don't check dispc timings for DSI

While most display types only forward their VM to the DISPC, this
is not true for DSI. DSI calculates the VM for DISPC based on its
own, but it's not identical. Actually the DSI VM is not even a valid
DISPC VM making this check fail. Let's restore the old behaviour
and avoid checking the DISPC VM for DSI here.

Fixes: 7c27fa57 ("drm/omap: Call dispc timings check operation directly")
Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>