drm: Fixed doc warnings in drm uapi header

Fixed doc warnings in drm uapi header. All the UAPI
structures are now documented in kernel doc.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1559159944-21103-4-git-send-email-uma.shankar@intel.com