drm/amd/display: Fix VBA chroma calculation for pipe splitting

[Why]
DML failures occur for 420 modes with dynamic pipe
splitting enabled because the ChromaViewport exceeds
the ChromaSurfaceWidth.

This is caused by adding the viewport_width instead
of the viewport_width_c.

This similarly occurs for rotated modes due to the
use of viewport_height instead of viewport_height_c.

[How]
Correct the calculations.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>