drm/i915: simplify context_idr_cleanup

The idr code already passes us the pointer associated with that id, so
no need to look it up again. Also, we'll kill the idr right away, so
there's no issue with leaving these dangling pointers behind - the
current code does the same.

v2: Also drop the file argument, spotted by Ben Widawsky.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>