drm: drm_atomic_helper.c: Correct comments format

Corrected comments format in accordance to the Linux style guides.

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210412124213.4628-3-fmdefrancesco@gmail.com