drm/i915: correct lock type in destroy

This is only relevant when using module unloading, and really only helps
get rid of a probably benign warning.

I can't remember if I sent this out already, but it's not turning up in
any of my searches.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>