drm/atomic: Add ->atomic_check() to encoder helpers

This callback can be used instead of the legacy ->mode_fixup() and is
passed the CRTC and connector states. It can thus use these states to
validate the modeset and cache values in the state to be used during
the actual modeset.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>