drm/i915/display: let intel_plane_uses_fence be used from other places.

I want to refactor some stuff using this so make it shared.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211012043502.1377715-3-airlied@gmail.com