tgsi_to_nir: Improve interpolation modes.

tgsi_to_nir: Improve interpolation modes.

This patch extracts the interpolation mode translation
into a separate function called ttn_translate_interp_mode,
adds support for TGSI_INTERPOLATE_COLOR which was missing,
and also sets the proper interpolation mode to output
variables, which were not set previously.

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>