drm/omap: verify that fb plane pitches are the same

The DSS hardware uses the same ROW_INC value for both Y and UV planes
for NV12 format. This means that the pitches of the Y and UV planes have
to match. omapdrm doesn't check this at the moment, and this can lead
into a broken NV12 fb on the screen.

This patch adds the check.

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