drm/bridge: Move devm_drm_of_get_bridge to bridge/panel.c

drm/bridge: Move devm_drm_of_get_bridge to bridge/panel.c

By depending on devm_drm_panel_bridge_add(), devm_drm_of_get_bridge()
introduces a circular dependency between the modules drm (where
devm_drm_of_get_bridge() ends up) and drm_kms_helper (where
devm_drm_panel_bridge_add() is).

Fix this by moving devm_drm_of_get_bridge() to bridge/panel.c and thus
drm_kms_helper.

Fixes: 336c3299149d ("drm/bridge: Add a function to abstract away panels")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210917180925.2602266-1-maxime@cerno.tech