This try to reset the dma engine when performing gpu reset. Hopefully
bringing back the gpu dma engine in sane state.

v2: agd5f: fix dma reset on cayman/TN, add support for SI
Signed-off-by: NJerome Glisse <jglisse@redhat.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>