media: test-drivers: Stop direct calls to queue num_buffers field

Use vb2_get_num_buffers() to avoid using queue num_buffers field directly.
This allows us to change how the number of buffers is computed in the
future.
If 'min_buffers_needed' is set remove useless checks in queue setup
functions.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
CC: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

media: media/test_drivers: rename to test-drivers

We never use _ in directory names in the media subsystem, so
rename to test-drivers instead for consistency.

Also update MAINTAINERS with the new path.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>