avutil/hwcontext_vulkan: disable host transfers if ReBAR is disabled

This feature fundamentally relies on host-visible VRAM, which restricts the
set of available memory types to (typically) host-visible device-local ones.

When resizable BAR is disabled, this memory type is usually limited to
e.g. 256 MiB in size, which is just plain insufficient for allocation of
general purpose GPU images, causing OOM errors on even the simplest of
commands.

The easiest solution is to disable host transfers entirely on machines
without host-addressable VRAM. In theory, we could try and recover the use
of host transfers for images which are *not* restricted to device-local
memory types, but this is rarely the case in practice, and the effort
required would exceed the benefit, especially since ReBAR is a standard
feature on all platforms recent enough to have Vulkan drivers, and only
occasionally disabled in the UEFI for by default for some hare-brained
notion of "backwards compatibiility" with ancient software.

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