intel: Account for potential pinned buffers hogging fences

As the kernel reports the total number of fences, we must guess how many
fences are likely to be pinned. In the typical system these will be only
used by the scanout buffers, of which there may be one per pipe, and any
number of manually pinned fenced buffers. So take a conservative guess
and reserve two fences for use by the system.

Note this reduces the number of fences to 3 for i915 and prior.

Reference:
  http://bugs.freedesktop.org/show_bug.cgi?id=25911
  The latest intel driver 2.10.0 causes kernel oops and system hangs

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

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