drm/i915: optimize vblank waits in set_base_atomic

We only need to do them if the pipe is actually running and if the
framebuffers have changed. Removes two "wait for vblank timed out"
messages when doing a suspend/resume cycle on my i855gm.

v2: s/to_intel_ctrc(crtc)/intel_crtc/ spotted by Chris.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>