drm/amdgpu: increase IH ring buffer size to avoid overflow

We originally limited the IH to 4k on tonga since it
uses bus addresses directly rather than GPU MC addresses,
so it needs contigous physical memory.  This brings it
inline with other asics.

Signed-off-by: Roger.He <Hongbo.He@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>