i915: Clean up sarea pointers on leavevt

This corresponds to the setup of the sarea pointers in DMA initialization,
though neither is exactly the point at which the sarea is set up or torn down.

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