drm/print: Update old comment style

Remove old comment style used by doxygen.
And remove comment left from commit 99cdb35e ("drm/doc: move printf
helpers out of drmP.h") after refactoring drmP.h.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/4d89eef3945dbffd402d2ecdc130108b0565c158.1508297716.git.hamohammed.sa@gmail.com