drm/panel: novatek-nt35950: Only unregister DSI1 if it exists

Commit 5dd45b66 ("drm/panel: novatek-nt35950: Improve error handling")
introduced logic to unregister DSI1 on any sort of probe failure, as
that's not done automatically by kernel APIs.

It did not however account for cases where only one DSI host is used.
Fix that.

Fixes: 5dd45b66 ("drm/panel: novatek-nt35950: Improve error handling")
Reported-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417-topic-maple_panel_fixup-v1-1-07c8db606f5e@linaro.org