drm/doc: re-format drm.h file comment

Our documentation build system chokes on \file comments:

    ./include/uapi/drm/drm.h:2: warning: Cannot understand  * \file drm.h
     on line 2 - I thought it was a doc line

Remove all of the slash-directives, and demote to a normal comment. Keep
the historical information because it predates Git.

v3: keep the comment (Daniel)

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201222133524.160842-4-contact@emersion.fr