drm/i915: Fix pid leak with banned clients

Get_pid_task() needs to be paired with a put_pid or we leak a pid
reference every time a banned client tries to create a context.

v2:
 * task_pid_nr helper exists! (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: b083a087 ("drm/i915: Add per client max context ban limit")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
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/20191217170933.8108-1-tvrtko.ursulin@linux.intel.com