d3d12: Add a path for mapping of not-directly-mappable buffers

d3d12: Add a path for mapping of not-directly-mappable buffers

Currently all buffers are allocated as mappable, but a future
commit will change that so that some buffers can be allocated
directly in non-CPU-accessible memory for improved performance.

Note that the returned pointer must be appropriately offset from
a 64-byte-aligned base pointer, so if offsets are used, the data
will be read/written to an offset region in the staging buffer.

Reviewed-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8095>