drm/amdgpu: refine the PTE encoding of PRT for navi10

Due to GCR change from navi10, the PTE encoding of PRT
needs change VSCTL = 01111 (was 0XX1X).

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