drm/bridge: panel: Remove a redundant check on existence of bridge->encoder

drm/bridge: panel: Remove a redundant check on existence of bridge->encoder

Because the existence of 'bridge->encoder' has already been checked before
the panel_bridge_attach() function get called, and the drm_bridge_attach()
will quit with a negative error code returned if it fails for some reasons.
Hence, it is guaranteed that the .encoder member of the drm_bridge instance
is not NULL when panel_bridge_attach() function get called.

Remove the redundant checking codes "if (!bridge->encoder) { ... }".

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240513153109.46786-5-sui.jingfeng@linux.dev