drm/i915: extract fbc_supported()

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: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>