drm/amdgpu/sdma5: fix wptr overwritten in ->get_wptr()

"u64 *wptr" points to the the wptr value in write back buffer and
"*wptr = (*wptr) >> 2;" results in the value being overwritten each time
when ->get_wptr() is called.

umr uses /sys/kernel/debug/dri/0/amdgpu_ring_sdma0 to get rptr/wptr and
decode ring content and it is affected by this issue.

fix and simplify the logic similar as sdma_v4_0_ring_get_wptr().

v2: fix for sdma5.2 as well

Suggested-by: Le Ma <le.ma@amd.com>
Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>