drm/i915/fbc: Allow FBC to recompress after a 3D workload on i85x/i865

Normally i85x/i865 3D activity will block FBC until a 2D blit
occurs. I suppose this was meant to avoid recompression while
3D activity is still going on but the frame hasn't yet been
presented. Unfortunately that also means that a page flipped
3D workload will permanently block FBC even if it only renders
a single frame and then does nothing.

Since we are using software render tracking anyway we might as
well flip the chicken bit so that 3D does not block FBC. This
will avoid the permament FBC blockage in the aforemention use
case, but thanks to the software tracking the compressor will
not disturb 3D rendering activity.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702153723.24327-5-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>