[media] mt9p031: Fix horizontal and vertical blanking configuration

Compute the horizontal blanking value according to the datasheet. The
value written to the hblank and vblank registers must be equal to the
number of blank columns and rows minus one.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>