drm/i915/bdw: Update MI_FLUSH_DW

The code is more verbose than necessary for the reader's sake, hopefully
the compiler optimizes away the if.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>