drm/amd/display: Allow subvp on vactive pipes that are 2560x1440@60

Enable subvp on specifically 1440p@60hz displays even though it can
switch in vactive.

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>