gallium: add BIND flags for R/W buffers and images

gallium: add BIND flags for R/W buffers and images

PIPE_CAPs and TGSI support will be added later. The TGSI support should be
straightforward. We only need to split TGSI_FILE_RESOURCE into TGSI_FILE_IMAGE
and TGSI_FILE_BUFFER, though duplicating all opcodes shouldn't be necessary.

The idea is:
* ARB_shader_image_load_store should use set_shader_images.
* ARB_shader_storage_buffer_object should use set_shader_buffers(slots 0..M-1)
  if M shader storage buffers are supported.
* ARB_shader_atomic_counters should use set_shader_buffers(slots M..N)
  if N-M+1 atomic counter buffers are supported.

PIPE_CAPs can describe various constraints for early DX11 hardware.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>