tgsi_to_nir: Support FACE and POSITION properly.

tgsi_to_nir: Support FACE and POSITION properly.

Previously, FACE was hard-coded as a sysval, but TTN emulated
it incorrectly. Also, POSITION was not supported when it was
a sysval. This patch fixes these by allowing both of them to
be sysvals or inputs, based on driver capabilities. It also
fixes the TGSI FACE emulation based on the TGSI spec.

Signed-Off-By: Timur Kristóf <timur.kristof@gmail.com>
Tested-by: Andre Heider <a.heider@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Eric Anholt <eric@anholt.net>