drm/kms: Add a module parameter to disable polling

Polling for a VGA device on an old system can be quite expensive,
causing latencies on the order of 600ms. As we hold the mode mutex for
this time and also need the same mutex to move the cursor, we trigger a
user-visible stall.

The real solution would involve improving the granulatity of the
locking and so perhaps performing some of the probing not under the lock
or some other updates can be done under different locks. Also reducing the
cost of probing for a non-existent monitor would be worthwhile. However,
exposing a parameter to disable polling is a simple workaround in the
meantime.

In order to accommodate users turning polling on and off at runtime, the
polling is potentially re-enabled on every probe. This is coupled to
the user calling xrandr, which seems to be a vaild time to reset the
polling timeout since the information on the connection has just been
updated. (The presumption being that all connections are probed in a
single xrandr pass, which is currently valid.)

References:

  Bug 29536 - 2.6.35 causes ~600ms latency every 10s
  https://bugs.freedesktop.org/show_bug.cgi?id=29536

  Bug 16265 - Why is kslowd accumulating so much CPU time?
  https://bugzilla.kernel.org/show_bug.cgi?id=16265



Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>