iris: Face should be a system value.
This patch adds PIPE_CAP_TGSI_FS_FACE_IS_INTEGER_SYSVAL which despite its name is not a TGSI-specific capability, just lets the state tracker know that it should generate a system value for FACE.
This is needed if we want to run tgsi_to_nir on iris.
Signed-off-by: Timur Kristóf timur.kristof@gmail.com