drm/i915/perf: ensure we keep a reference on the driver

The i915 perf stream has its own file descriptor and is tied to
reference of the driver. We haven't taken care of keep the driver
alive.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: eec688e1 ("drm/i915: Add i915 perf infrastructure")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190709123351.5645-2-lionel.g.landwerlin@intel.com