drm/amdgpu: Clean sdma wptr register when only enable wptr polling
The sdma wptr polling memory is not fast enough, then the sdma
wptr register will be random, and not equal to sdma rptr, which
will cause sdma engine hang when load driver, so clean up the sdma
wptr directly to fix this issue.
v2:add comment above the code and correct coding style
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>