drm/radeon: Always flush the VM

This is slightly cleaned up version of Jerome's patch.
There seems to be an issue tracking the last flush of
the VM which results in hangs in certain cases when
VM is used.  For now just flush the VM for every IB.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=62959
https://bugs.freedesktop.org/show_bug.cgi?id=62997



Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org