drm/i915: Remove use of runtime pm in atomic commit functions

We needed this originally for updating pagetables in plane commit
functions. But that's extracted into prepare/cleanup now. The other
issue was running updates when the pipe was off. That's also now
fixed.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>