drm/amdgpu: Set PTE_IS_PTE bit for gfx12

Set PTE_IS_PTE bit while PRT is enabled on gfx12.

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