amdkfd: MTYPE_UC for ext-coherent system memory

Set memory mtype to UC host memory when ext-coherent
flag is set and memory is registered as a SVM allocation.

Reviewed-by: Amber Lin <Amber.Lin@amd.com>
Signed-off-by: David Yat Sin <David.YatSin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 5d14fdab)