drm/amd/display: Fix arithmetic error in MALL size calculations for subvp

[Description]
Need to subtract unused section of the viewport when calculating required space
in MALL for subvp instead of adding, to prevent over allocation.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>