drm/nouveau: add nv_encoder pointer check for NULL

Pointer nv_encoder could be dereferenced at nouveau_connector.c
in case it's equal to NULL by jumping to goto label.
This patch adds a NULL-check to avoid it.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 3195c5f9

 ("drm/nouveau: set encoder for lvds")
Signed-off-by: Natalia Petrova <n.petrova@fintech.ru>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[Fixed patch title]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512103320.82234-1-n.petrova@fintech.ru