drm/nouveau/dp: check for NULL nv_connector->native_mode

Add checking for NULL before calling nouveau_connector_detect_depth() in
nouveau_connector_get_modes() function because nv_connector->native_mode
could be dereferenced there since connector pointer passed to
nouveau_connector_detect_depth() and the same value of
nv_connector->native_mode is used there.

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

Fixes: d4c2c99b

 ("drm/nouveau/dp: remove broken display depth function, use the improved one")

Signed-off-by: Natalia Petrova <n.petrova@fintech.ru>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512111526.82408-1-n.petrova@fintech.ru