drm/i915: Avoid reading fbc registers in vain when fbc was never enabled.

If it wasn't never enabled by kernel parameter or platform default
we can avoid reading registers so many times in vain

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