drm/amdgpu: Add RREG64_PCIE_EXT/WREG64_PCIE_EXT functions

Add 64bits register access support on register whose address
is greater than 32bits.

Signed-off-by: Candice Li <candice.li@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>