drm/amd/display: fix double assignment to msg_id field

The msg_id field is being assigned twice. Fix this by replacing the second
assignment with an assignment to msg_size.

Addresses-Coverity: ("Unused value")
Fixes: 11a00965 ("drm/amd/display: Add PSP block to verify HDCP2.2 steps")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>