drm/bridge: ti-sn65dsi86: use dev name for debugfs

This should be more future-proof if we ever encounter a device with two
of these bridges.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190706203105.7810-1-robdclark@gmail.com