drm/radeon: enable display scaling on all connectors (v2)

This enables the display scaler on all connectors for r5xx
and newer asics.  Previously we only enabled the scaler for
fixed mode displays (eDP or LVDS) since they have to use the
scaler to support non-native modes.  Most other displays
are multi-sync or have a built in scaler to support non-native
modes.  The default scaling mode for non-fixed displays is
none which will use the scaler in the monitor.  Note that
we do not populate any fake modes like we do for fixed
displays so it will only use the modes in the edid.  For
other modes, you'll need to populate them manually.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=80868

v2: properly handle scaling with no modes defined

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>