drm/i915: rip out the overlay pipe A workaround

Now that all affected i830M systems have the pipe A quirk set,
we don't need to do any special dances in the overlay code any
longer. And reading through the code I'm rather dubios that it
actually does what it claims to do ...

As a nice benefit this rips out a users of the crtc helper dpms
callback.

v2: As suggested by Chris Wilson, replace the code by an appropriate
WARN to ensure that the pipe A is indeed running.

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