This patch fixes two issues during media manager registration.

1. The ppa pool can be used at media manager registration. Allocate the
ppa pool before that.

2. If a media manager can't be found, this should not lead to the
device being unallocated. A media manager can be registered later, that
can manage the device. Only warn if a media manager fails
initialization.
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>