drm: use correct dev node location in comment

Current comment mentions /dev/drm which hasn't been a thing even before
the code was merged into the kernel ;-)

v2: drop explicit node path (Sean)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722165648.7828-1-emil.l.velikov@gmail.com