Round off image width to nearest 8 (GTF_CELL_GRAN)

A source sending a GTF (Generalized Timing Formula) format have no means of
signalling image width. The assumed aspect ratio may result in an odd image
width but according to the standard image width should be in multiple of 8.

Cc: Mats Randgaard <matrandg@cisco.com>
Signed-off-by: Martin Bugge <marbugge@cisco.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>