drm/amdgpu: validate wait_for_idle before function call

Before making a function call to wait_for_idle,
validate the function pointer like we do in sw_init.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>