program: Extend prog_to_nir handle system values.
Some drivers, such as radeonsi, use a system value for gl_FragCoord rather than an input variable. In this case, our Mesa IR will have a PROGRAM_SYSTEM_VALUE register, which we need to translate. This makes prog_to_nir work for Gallium drivers which expose the PIPE_CAP_TGSI_FS_POSITION_IS_SYSVAL capability bit. Reviewed-by: Eric Anholt <email@example.com>