gallium/u_threaded: late alloc cpu_storage

gallium/u_threaded: late alloc cpu_storage

Instead of allocating cpu_storage in threaded_resource_init, defer the
allocation to first use (in tc_buffer_map).
This avoids needless memory allocation if tc_buffer_disable_cpu_storage is
called before tc_buffer_map.

map_buffer_alignment is stored and serves as a "can cpu_storage be used" flag.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15074>