drm/i915: Filter out no-op frontbuffer tracking flushes

Paulo noticed that the fbc frontbuffer tracking flush callback
occasionally gets a call without any bit set. This can happen when we
have to filter flush calls due to e.g. gpu rendering. Filter these
out.

Reported-by: Paulo Zanoni <przanoni@gmail.com>
Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>