i965/vec4: use vec4_builder to emit instructions in setup_imm_df()

i965/vec4: use vec4_builder to emit instructions in setup_imm_df()

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
[ Francisco Jerez: Drop useless vec4_visitor dependencies.  Demote to
  static stand-alone function.  Don't write unused components in the
  result.  Use vec4_builder interface for register allocation. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>