drm/amdgpu: implement UMC 64 bits REG operations

implement 64 bits operations via 32 bits interface

v2: make use of lower_32_bits() and upper_32_bits() macros

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>