drm/amd/display: Fix HDMI VSIF V3 incorrect issue

[Why]
Reported from customer the checksum in AMD VSIF V3 is incorrect and
causing blank screen issue.

[How]
Fix the packet length issue on AMD HDMI VSIF V3.

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Leo Ma <hanghong.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>