drm/amdgpu/gmc9: disable partial wr rmw if ECC is not enabled

The vbios mistakenly sets this bit on some boards without ECC.
This can lead to reduced performance in some workloads.  Disable
the bit if the board does not have ECC.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>