media: mediatek: vcodec: Fix possible invalid memory access for encoder

The vpu maybe null pointer or unreasonable value when scp crash, need to
validate that the vpu pointer and the vpu instance within this context is
valid in case of leading to kernel reboot.

Fixes: 27a274db

 ("[media] vcodec: mediatek: Add Mediatek VP8 Video Encoder Driver")
Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reported-by: Steve Cho <stevecho@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>