media: mediatek: vcodec: adding lock to protect encoder context list

Add a lock for the ctx_list, to avoid accessing a NULL pointer
within the 'vpu_enc_ipi_handler' function when the ctx_list has
been deleted due to an unexpected behavior on the SCP IP block.

Fixes: 1972e324

 ("media: mediatek: vcodec: Fix possible invalid memory access for encoder")
Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>