Without this change, blits to the front buffer won't invalidate FBC
state, causing us to scan out stale data.  Make sure we update these
bits on every FBC enable, since they may get clobbered if we shut off
the display.

References: https://bugzilla.kernel.org/show_bug.cgi?id=26932Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>