drm/amdgpu: Update the impelmentation of AMDGPU_PTE_MTYPE_GFX12

This patch changes the implementation of AMDGPU_PTE_MTYPE_GFX12,
clear the bits before setting the new one.
This fixed the potential issue that GFX12 setting memory to NC.

v2: Clear mtype field before setting the new one (Alex)
v3: Fix typo (Felix)

Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: longlyao <Longlong.Yao@amd.com>
Signed-off-by: Shane Xiao <shane.xiao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>