drm/amd/display: Use viewport height for subvp mall allocation size

[WHY?]
MALL allocation size depends on the viewport height, not the addressable
vertical lines, which will not match when scaling.

[HOW?]
Base MALL allocation size calculations off viewport height.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>