drm/amdgpu: Correct dGPU MTYPE settings for gfx943

Revert temporary dGPU VRAM MTYPE setting and align with expected
coherency protocol.

Signed-off-by: Graham Sider <Graham.Sider@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>