drm/amd/display: Check index msg_id before read or write

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Check index msg_id before read or write

[WHAT]

msg_id is used as an array index and it cannot be a negative value, and

therefore cannot be equal to MOD_HDCP_MESSAGE_ID_INVALID (-1).

[HOW]

Check whether msg_id is valid before reading and setting.

This fixes 4 OVERRUN issues reported by Coverity.