drm/amdgpu: Add mem_sync implementation for all the ASICs.

Implement the .mem_sync hook defined earlier.

v2: Rename functions

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>