drm: Fix a couple of typos, punctation and whitespace issues

These are just a couple of things that I came across as I was reading
through the code and comments.

v2: added one more hunk that ended up in the wrong patch

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206135336.2084564-1-thierry.reding@gmail.com