drm: Add atomic variants for bridge enable/disable

This patch adds atomic variants for all of
pre_enable/enable/disable/post_disable bridge functions. These will be
called from the appropriate atomic helper functions. If the bridge
driver doesn't implement the atomic version of the function, we will
fall back to the vanilla implementation.

Note that some drivers call drm_bridge_disable directly, and these cases
are not covered. It's up to the driver to decide whether to implement
both atomic_disable and disable, or if it's not necessary.

Changes in v3:
- Added to the patchset
Changes in v4:
- Fix up docbook references (Daniel)
Changes in v5:
- None

Link to v3: https://patchwork.freedesktop.org/patch/msgid/20190502194956.218441-4-sean@poorly.run
Link to v4: https://patchwork.freedesktop.org/patch/msgid/20190508160920.144739-4-sean@poorly.run



Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611160844.257498-4-sean@poorly.run