drm/i915: Move load time runtime PM get later

We require the device to be powered only before accessing it, so we can
move this call later.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458128348-15730-9-git-send-email-imre.deak@intel.com