drm/i915: Subtract total pinned bytes from available aperture size

The old code was wandering through the active list looking for pinned
buffers; there may be other pinned buffers around. Fortunately, we keep a
count of the total amount of pinned memory and can use that instead.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>