drm/omap: verify that display x-res is divisible by 8

DISPC requires the x resolution to be divisible by 8 when stall mode is
not used.

Add a check to the DPI driver to verify this.

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