drm/i915: Clean up scaler setup, v2.

On skylake we can switch to a high quality scaler mode when only 1 out
of 2 scalers are used, but on GLK and later bit 28 has a different
meaning. Don't set it, and make clear the distinction between
SKL and later PS values.

Changes since v1:
- Add missing break statement.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
[mlankhorst: Remove extra newline]
Link: https://patchwork.freedesktop.org/patch/msgid/20180921144437.20037-1-maarten.lankhorst@linux.intel.com