drm/amd: Disable XNACK on SRIOV environment

The purpose of this patch is to disable XNACK or set XNACK OFF mode
on SRIOV platform which doesn't support it.

This will prevent user-space application to fail or result into
unexpected behaviour whenever the application need to run test-case
in XNACK ON mode.

Signed-off-by: Surbhi Kakarya <surbhi.kakarya@amd.com>
Reviewed-by: Shaoyun Liu <shaoyun.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>