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.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>