drm/atomic-helper: Make atomic_enable/disable crtc callbacks optional

drm/atomic-helper: Make atomic_enable/disable crtc callbacks optional

Allow atomic_enable and atomic_disable operations from
drm_crtc_helper_funcs struct optional. With this, the target display
drivers don't need to define a dummy function if they don't need one.

Changes since v2:
* Don't make funcs optional
* Update kerneldoc for atomic_enable/disable
* Replace "if (funcs->atomic_enable)" by "if (funcs->commit)"
* Improve commit message

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190314184845.gjmvkamobj4dilyp@smtp.gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>