drm/i915: check that we are in an RPM atomic section in GGTT PTE updaters

The device should be on for the whole duration of the update, so check
for this.

v2:
- use the existing dev_priv directly everywhere (Ville)
v3:
- check also that we are in an RPM atomic section (Chris)
- add the assert to i915_ggtt_insert_entries/clear_range too (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/1450203038-5150-11-git-send-email-imre.deak@intel.com