ui/hob: Force the 'Base image combo' to be drawn correctly

ui/hob: Force the 'Base image combo' to be drawn correctly

As the combo is created before its backing model it's common for the combo to
be drawn at its minimum size and then grow the first time the user activates
it. This slight ugly patch forces the combo to be resized as soon as the
model is associated so that by the time the user interacts with the widget it
is less likely to change size.

(Bitbake rev: 65819447ccc96ae2af8d42cf3a31769ef3d2d26e)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>