drm/amd/display: correct dcn21 NUM_VMID to 16

1 vmid limitation only exists for HOSTVM which is a custom
use case anyway.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>