drm/amdgpu: Restore uncached behaviour on GFX12

Always use MTYPE_UC if UNCACHED flag is specified.

This makes kernarg region uncached and it restores
usermode cache disable debug flag functionality.

Do not set MTYPE_UC for COHERENT flag, on GFX12 coherence is handled by
shader code.

Signed-off-by: David Belanger <david.belanger@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>