drm/i915: Remove unwanted rcu_read_lock/unlock

We don't need rcu read side critical section to call pid_nr(),
remove it.

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@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/20191106172416.17188-1-chris@chris-wilson.co.uk