Make it clear that we're checking whether FBC is supported or not. The
fact that the vfunc is not NULL is just a consequence.

Another name option would have been fbc_initialized().
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>