drm/i915: get runtime PM reference around GEM set_caching IOCTL

drm/i915: get runtime PM reference around GEM set_caching IOCTL

After Damien's D3 fix I started to get runtime suspend residency for the
first time and that revealed a breakage on the set_caching IOCTL path
that accesses the HW but doesn't take an RPM ref. Fix this up.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: stable@vger.kernel.org
Link: http://patchwork.freedesktop.org/patch/msgid/1446665132-22491-1-git-send-email-imre.deak@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>