drm/i915: Remove unnecessary test on the gen in intel_do_mmio_flip()

use_mmio_flip() makes sure we only enable MMIO flips on gen5+. So we
don't need to take into account older devices.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>