Due to the upcoming atomic modesetting feature we need to separate
some update functions into a check step that can fail and a commit
step that should, ideally, never fail.

The commit part can still fail, but that should be solved in another
upcoming patch.
Signed-off-by: NGustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>