drm: Tweak drm_encoder_helper_funcs.enable kerneldoc

drm: Tweak drm_encoder_helper_funcs.enable kerneldoc

I copied the kerneldoc for encoder_funcs.atomic_enable from encoder_funcs.enable
in a recent patch [1]. Sam rightly pointed out in the review that "for symmetry
with" text is awkward [2]. So here's a patch to fix up the source of the awkward
language.

[1] https://patchwork.freedesktop.org/patch/msgid/20190611160844.257498-2-sean@poorly.run
[2] https://patchwork.freedesktop.org/patch/msgid/20190611185352.GA16305@ravnborg.org

Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190612150038.194843-1-sean@poorly.run