media: i2c: Add hblank control to ov7251

Add a hblank control to the ov7251 driver. This necessitates setting
a default mode, for which I am simply picking the first available.

Signed-off-by: Daniel Scally <djrscally@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>