drm/amdkfd: Don't drain ih1 for APU

ih1 is not initialized for APUs. Don't drain it or NULL pointer
error will be triggered.

Fixes: 6ef29715 ("drm/amdkfd: Change kfd/svm page fault drain handling")
Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>