drm/amdgpu: remove exp hardware support from iceland

It's working now.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=92270



Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org