drm/amdgpu: Clean sdma wptr register when only enable wptr polling

drm/amdgpu: Clean sdma wptr register when only enable wptr polling

BugLink: http://bugs.launchpad.net/bugs/1786352
[ Upstream commit 4062119b9d958df33bcda703dc3ac646908fa861 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>