media: mediatek: vcodec: Handle invalid encoder vsi

[ Upstream commit 19e2e01f ]

Handle invalid encoder vsi in vpu_enc_init to ensure the encoder
vsi is valid for future use.

Fixes: 1972e324 ("media: mediatek: vcodec: Fix possible invalid memory access for encoder")

Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sasha Levin <sashal@kernel.org>