drm/omap: fix setting & clearing DOUBLESTRIDE

The code that sets and clears DOUBLESTRIDE is only ran when using NV12.
This is not correct, as we might first set the bith when using NV12, but
never clear it when using other formats.

Fix it so that when the bit is available (when the HW supports NV12) we
always either set or clear the bit.

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