drm/i915/fbc: don't try to deactivate FBC if it's not enabled

During FBC invalidation, don't call intel_fbc_deactivate if it's not
enabled. This doesn't fix any bug, but helps making the interface
saner.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-21-git-send-email-paulo.r.zanoni@intel.com