drm/amd/display: fix up HUBBUB hw programming for VM

[why]
Some values were not being converted or bit-shifted properly for
HW registers, causing black screen

[how]
Fix up the values before programming HW

Signed-off-by: Jun Lei <jun.lei@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>