drm/i915/debugfs: Take runtime_pm ref for sseu

drm/i915/debugfs: Take runtime_pm ref for sseu

When reading the SSEU statistics, we need to call
intel_runtime_pm_get() first, otherwise we might end up
triggering "Device suspended during HW access".

Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470062007-26996-1-git-send-email-david.weinehall@linux.intel.com
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>