[media] v4l2-dv-timing: avoid rounding twice in gtf hblank calc

Currently, in gtf hblank calculations, the rounding is used twice,
one at intermediate division and one at final state where hblank
is rounded to nearest multiple of twice cell granularity. This
error got introduced in 'commit d7ed5a3d ("[media]
v4l2-dv-timings: fix rounding in hblank and hsync calculation"),
where it missed combining the rounding step. Correcting the same
in this patch.

Signed-off-by: Prashant Laddha <prladdha@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>