drm/amdgpu/gfx: Improvement on EDC GPR workarounds

SPI limits total CS waves in flight per SE to no more than 32 * num_cu and
we need to stuff 40 waves on a CU to completely clean the SGPR. This is
accomplished in the WR by cleaning the SE in two steps, half of the CU per
step.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>