The ctx_list will be deleted when scp getting unexpected behavior, then the
ctx_list->next will be NULL, the kernel driver maybe access NULL pointer in
function vpu_enc_ipi_handler when going through each context, then reboot.
Need to add lock to protect the ctx_list to make sure the ctx_list->next isn't
NULL pointer.
Fixes: 1972e32431ed ("media: mediatek: vcodec: Fix possible invalid memory access for encoder")
Signed-off-by: Yunfei Dong <yunfei.dong@xxxxxxxxxxxx>