drm/i915: Call runtime_pm_disable directly

Allows us to mark it static and so forgoe the kerneldoc for it.

Note that intel_power_domains_fini is also called from failure paths
in the driver load sequence. But the call to runtime_pm_disable for
that is harmless since by default runtime pm is already disabled.

v2: Augment the commit message as discussed with Imre on irc.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>