drm/amdgpu: fix bug with IH ring setup

The bug limits the IH ring wptr address to 40bit. When the system memory
is bigger than 1TB, the bus address is more than 40bit, this causes the
interrupt cannot be handled and cleared correctly.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>