drm/i915: don't keep reassigning FBC_UNSUPPORTED

This may save a few picoseconds on !HAS_FBC platforms. And it also
satisfies my OCD.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>