drm/omap: omap_display_timings: rename vfp to vfront_porch

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

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