drm/msm: add special _get_vaddr_active() for cmdstream dumps

Prep work for adding a debugfs file that dumps just submits which
trigger hangs/faults.  In this case the bo may already be in the
MADV_DONTNEED state, but will be still on the active list (since
the submit hasn't completed yet).  So the normal check that the
bo is in the WILLNEED state does not apply.  (But of course the bo
should definitely not be in the PURGED state!)

Signed-off-by: Rob Clark <robdclark@gmail.com>