For gfx ring only.  Compute is still todo.

v2: add documentation
v3: update to latest reset changes, integrate emit update patch.
v4: fix count on wait_reg_mem for HDP flush
v5: use old hdp flush method for fence
v6: set valid bit for IB
v7: cleanup for release
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>