drm/i915: Make intel_format_to_fourcc() static

v2: Fix conflict caused by the introduction of struct intel_crtc_state

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>