drm/i915: Fix PSF GV point mask when SAGV is not possible

Don't just mask off all the PSF GV points when SAGV gets disabled.
This should in fact cause the Pcode to reject the request since
at least one PSF point must remain enabled at all times.

Cc: stable@vger.kernel.org
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: 192fbfb7

 ("drm/i915: Implement PSF GV point support")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220309164948.10671-7-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>