drm: fix kernel-doc warnings in drm_crtc.h

Fix the following 'make htmldocs' warning:

  .//include/drm/drm_crtc.h:929: warning: Excess struct/union/enum/typedef member 'base' description in 'drm_bridge'

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>