drm/i915: refactor RPM disabling due to RC6 being disabled

We can make the RPM dependency on RC6 explcit in the code by taking an
actual RPM reference, instead of avoiding to drop the initial one. This
will also enable us to remove the HAS_RUNTIME_PM special casing from
more places in the next patch.

v2:
- fixed typo in commit message (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/1450203038-5150-4-git-send-email-imre.deak@intel.com