drm/i915: Do not add all planes when checking scalers on glk+

drm/i915: Do not add all planes when checking scalers on glk+

We cannot switch between HQ and normal mode on GLK+, so only
add planes on platforms where it makes sense.

We could probably restrict it even more to only add when scaler
users toggles between 1 and 2, but lets just leave it for now.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920114235.22411-9-maarten.lankhorst@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>