drm/omap: omap_display_timings: rename y_res to vactive

In preparation to move the stack to use the generic videmode struct for
display timing information rename the y_res member to vactive.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>