drm/atomic-helper: Add a note in drm_atomic_helper_reset_crtc() kernel-doc

As suggested in [0], add a note indicating that
drm_atomic_helper_reset_crtc() can be a no-op in some cases.

[0]:https://lore.kernel.org/all/Z7XfnPGDYspwG42y@phenom.ffwll.local/



Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20250220140406.593314-1-herve.codina@bootlin.com


Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>