drm/i915: fix FBC frontbuffer tracking flushing code

Due to the way busy_bits was handled, we were not doing any flushes if
we didn't previously get an invalidate. Since it's possible to get
flushes without an invalidate first, remove the busy_bits early
return.

So now that we don't have the busy_bits guard anymore we'll need the
origin check for the GTT tracking (we were not doing anything on GTT
flushes due to the GTT check at invalidate()).

As a last detail, since we can get multiple consecutive flushes,
disable FBC before updating it, otherwise intel_fbc_update() will just
keep FBC enabled instead of restarting it.

Notice that this does not fix any of the current IGT tests due to the
fact that we still have a few intel_fbc() calls at points where we
also have the frontbuffer tracking calls: we didn't fully convert to
frontbuffer tracking yet. Once we remove those calls and start relying
only on the frontbuffer tracking infrastructure we'll need this patch.

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