drm: simple_kms_helper: add support for bridges

drm: simple_kms_helper: add support for bridges

Introduce drm_simple_display_pipe_attach_bridge() and
drm_simple_display_pipe_detach_bridge() in order to make it possible to use
drm encoders with the simple display pipes managed by simple_kms_helpers

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1472115874-6219-3-git-send-email-andrea.merello@gmail.com