drm/i915/fbc: Just use params->fence_y_offset always

No need to tiptoe around programming DPFC_FENCE_YOFF with
params->fence_y_offset vs. 0. If the fence is not enabled
it doesn't even matter what we program here.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-4-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>