drm: improve kernel-docs in drm_mode.h

- Remove duplicate doc-comments for struct members
- Add missing @member markers for in-line member comments

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/grZIqIAOSUM7eNL0PurBsaWoILFwN2hEKd40Ylgzg@cp7-web-041.plabs.ch