drm/bridge: panel: use drm_bridge_is_panel() instead of open code

drm/bridge: panel: use drm_bridge_is_panel() instead of open code

drm_panel_bridge_remove() reads bridge->funcs to find out whether this is a
panel bridge or another kind of bridge. drm_bridge_is_panel() is made
exactly for that, so use it.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250214-drm-assorted-cleanups-v7-3-88ca5827d7af@bootlin.com