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.