drm/amd/display: correct image viewport calculation

[why]
We didn't transfer the camera/video viewport coordinate
when doing rotation and mirror.

[how]
To correct the viewport coordinate in calculate_viewport().

Signed-off-by: Martin Tsai <Martin.Tsai@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>