drm/amd/display: Add array index check for hdcp ddc access

[Why]
Coverity reports OVERRUN warning. Do not check if array
index valid.

[How]
Check msg_id valid and valid array index.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>