drm/i915: WARN if the core runtime PM get helpers fail

We don't expect the core runtime PM get helpers to return any error, so
add a WARN for this. Also print the return value for all the callsites
to help debugging.

v2:
- Don't call pm_runtime_get_sync() as part of initing locals. (Chris)

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/1490693935-12638-1-git-send-email-imre.deak@intel.com