drm/i915/fbc: only update no_fbc_reason when active

In our snb farm in CI we have plenty of underruns, but not enough
stolen memory to enable fbc. Which means every time there's an
underrun the no_fbc_reason swichtes to something that makes
kms_frontbuffer_tracking fail instead of skip, adding massive amounts
of additional noise to igt test runs.

Make sure we don't try to disable fbc when it's off already.

v2: Squash in additional WARN_ON suggestion from Chris.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811072327.4335-1-daniel.vetter@ffwll.ch