iris: Face should be a system value.

This patch adds support for PIPE_CAP_TGSI_FS_FACE_IS_INTEGER_SYSVAL which
despite its name is not a TGSI-specific capability, just lets us know that
we should generate a system value for face.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
25 jobs for iris-face in 35 minutes and 15 seconds (queued for 38 seconds)