drm: Extract drm_bridge.h

We don't want to burry the bridge structures kerneldoc in drm_crtc.h.

Cc: Archit Taneja <archit.taneja@gmail.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160831160913.12991-3-daniel.vetter@ffwll.ch