drm/panel: novatek-nt35950: Improve error handling

In a very peculiar case when probing and registering with the secondary
DSI host succeeds, but the OF backlight or DSI attachment fails, the
primary DSI device is automatically cleaned up, but the secondary one
is not, leading to -EEXIST when the driver core tries to handle
-EPROBE_DEFER.

Unregister the DSI1 device manually on failure to prevent that.

Fixes: 623a3531 ("drm/panel: Add driver for Novatek NT35950 DSI DriverIC panels")
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230415-konrad-longbois-next-v1-1-ce695dc9df84@linaro.org