drm/omap: increase vblank wait timeout

omap_crtc_wait_pending() waits until the config changes have been taken
into use, usually at next vblank. The wait-timeout used is 50ms, which
usually is enough, but in some rare cases not.

As time wait-timeout is just a safety measure for cases where something
is broken, we can just as well increase the timeout considerably.

This patch makes the timeout 250ms.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>