drm/amd/display: Use scaling for non-native resolutions on LVDS

[Why]
Common resolutions are added to supported modes to enable compatibility
scenarios that compositors may use to do things like clone displays. There
is no guarantee however that the panel will natively support these modes.

[How]
If the compositor hasn't enabled scaling but a non-native resolution has
been picked for an LVDS panel turn the scaler on anyway.  This will ensure
compatibility.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>