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>

drivers/gpu/drm/drm_atomic_helper.c

@@ -1034,7 +1034,7 @@ disable_outputs(struct drm_device *dev, struct drm_atomic_state *old_state)
 			funcs->atomic_disable(crtc, old_crtc_state);
 		else if (funcs->disable)
 			funcs->disable(crtc);
-		else
+		else if (funcs->dpms)
 			funcs->dpms(crtc, DRM_MODE_DPMS_OFF);
 
 		if (!(dev->irq_enabled && dev->num_crtcs))
@@ -1277,10 +1277,9 @@ void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
 		if (new_crtc_state->enable) {
 			DRM_DEBUG_ATOMIC("enabling [CRTC:%d:%s]\n",
 					 crtc->base.id, crtc->name);
-
 			if (funcs->atomic_enable)
 				funcs->atomic_enable(crtc, old_crtc_state);
-			else
+			else if (funcs->commit)
 				funcs->commit(crtc);
 		}
 	}

include/drm/drm_modeset_helper_vtables.h

@@ -418,6 +418,8 @@ struct drm_crtc_helper_funcs {
 	 * Drivers can use the @old_crtc_state input parameter if the operations
 	 * needed to enable the CRTC don't depend solely on the new state but
 	 * also on the transition between the old state and the new state.
+	 *
+	 * This function is optional.
 	 */
 	void (*atomic_enable)(struct drm_crtc *crtc,
 			      struct drm_crtc_state *old_crtc_state);
@@ -441,6 +443,8 @@ struct drm_crtc_helper_funcs {
 	 * parameter @old_crtc_state which could be used to access the old
 	 * state. Atomic drivers should consider to use this one instead
 	 * of @disable.
+	 *
+	 * This function is optional.
 	 */
 	void (*atomic_disable)(struct drm_crtc *crtc,
 			       struct drm_crtc_state *old_crtc_state);