drm/drv: Use // for comments in example code

This improves Sphinx output in two ways:

- It avoids an unmatched single-quote ('), about which Sphinx complained:

    Documentation/gpu/drm-internals.rst:298:
    WARNING: Could not lex literal_block as "c". Highlighting skipped.

  An alternative approach would be to replace "can't" with a word that
  doesn't have a single-quote.

- It lets Sphinx format the comments in italics and grey, making the
  code slightly easier to read.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Daniel Vetter <daniel@ffwll.ch> [via irc]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808163629.14280-1-j.neuschaefer@gmx.net