radv: Don't underflow non-visible VRAM size.

radv: Don't underflow non-visible VRAM size.

In some APU situations the reported visible size can be larger than
VRAM size. This properly clamps the value.

Surprisingly both CTS and spec seem to allow a heap type with size 0,
so this seemed like the easiest option to me.

Signed-off-by: Bas Nieuwenhuizen <basni@google.com>
Fixes: 4ae84efbc5c "radv: Use enum for memory heaps."
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>