drm/omap: omap_display_timings: rename x_res to hactive

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

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