drm: meson: global clean-up

This patch aims to:
- Add general and TODO comments
- Respect coding style for multi-line comments
- Align macro definitions
- Remove useless macro

Signed-off-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/86pnn382e8.fsf@baylibre.com