program: Extend prog_to_nir handle system values.

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 <eric@anholt.net>