media: vivid: fix control handler mutex deadlock

vivid_update_format_cap() can be called from an s_ctrl callback.
In that case (keep_controls == true) no control framework functions
can be called that take the control handler mutex.

The new call to v4l2_ctrl_modify_dimensions() did exactly that.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 6bc7643d (media: vivid: add pixel_array test control)
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>