drm/radeon/ss: use num_crtc rather than hardcoded 6

commit 5317670692f61675394db2eb6713484b67383750 upstream.

When checking if a pll is in use.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>