drm: drm_print.h: fix kernel-doc markups

A kernel-doc markup should start with the identifier on its
first line.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/5b76c5625709aaaa3abee98faa620b9f3d27ff85.1603791716.git.mchehab+huawei@kernel.org