Add the HBLANK control as read-only.

The hblank value is fixed in the mode definition and is updated
everytime a new format is applied.
Signed-off-by: NJacopo Mondi <jacopo@jmondi.org>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@kernel.org>