i965: Implement ARB_shader_stencil_export (gen9+)

i965: Implement ARB_shader_stencil_export (gen9+)

v2: remove useless source_stencil_to_render_target (Ken)
Squash in the actual packing function, which also got to
  v2:
Move the definition of the OPCODE outside of FB_WRITE opcodes (Matt)
Reorder the regioning to be in VWH order (Matt)
Don't retype src in the backend, just assert instead (Matt)
Rename the debug prints to something better (Matt)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>