drm/i915/gen9: Avoid using negative array index in skl_update_plane()

scaler_id may be negative as shown by conditions later in the function,
so don't use it as an array index in that case.

v2:
- Remove ps_ctrl while at it (Ville).

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463059132-1720-2-git-send-email-imre.deak@intel.com