d3d12: Don't allocate mappable textures

d3d12: Don't allocate mappable textures

There's not really a reason to directly map textures. Doing so
requires the texture to be allocated in system RAM instead of
video RAM, which means all GPU access to it would be needlessly slow.

Notably, the one texture type that was allocated this way is the
display target texture for the software driver path. Instead, use
pipe_transfer_map to be able to copy the texture to system RAM.

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