iris: Face should be a system value.

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>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>