nv50/ir: use s[] addr space for shared buffers

nv50/ir: use s[] addr space for shared buffers

Shared memory address space (FILE_MEMORY_SHARED) must be used instead
of global memory when a shared memory area is declared.

Changes from v2:
 - oops, do not remove TGSI_FILE_BUFFER in a switch in
   nv50_ir_from_tgsi.cpp

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>