pipe_config is the new dev_priv!

More seriously, this is actually better since a pipe_config can be
thrown away if the modeset compute config stage fails. Whereas any
state stored in dev_prive needs to be painstakingly restored, since
otherwise a dpms off/on will wreak massive havoc. Yes, that even
applies to state only used in ->mode_set callbacks, since we need to
call those even for dpms on when the Haswell power well cleared
everything out.
Reviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>