drm/i915: check that we hold an RPM wakelock ref before we put it

With this change we have the corresponding wake lock checks in both the
rpm get and put functions.

v2-v3:
- unchanged
v4:
- keep the corresponding check in the get helper (Chris)
v5:
- add a note to the commit message that with this change we have the
  checks both in the rpm get and put functions (Joonas)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450203038-5150-9-git-send-email-imre.deak@intel.com