drm/amdkfd: Remove the requirement for atomic Ops on vg20

Firmware have the workaround to replace the atomic Ops with read-modify-write on CP side.
User should not expect atomic Ops on system memory works normally if system didn't not
support it.

Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-By: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>