drm/atomic: kerneldoc for drm_atomic_crtc_needs_modeset

Just a bit of drive-by ocd.

v2: Improve per Liviu's feedback.

Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-7-git-send-email-daniel.vetter@ffwll.ch