radeonsi: use SPI_SHADER_COL_FORMAT fields instead of export_16bpc

radeonsi: use SPI_SHADER_COL_FORMAT fields instead of export_16bpc

This does change the behavior slightly:
  If a shader writes COLOR[i] and that color buffer isn't bound,
  the shader will export MRT_NULL instead and discard the IR tree that
  calculates the output. The only exception is alpha-to-coverage, which
  requires an alpha export.

v2: - update a comment about 16BPC
    - account for MRTZ when when fixing alpha-test/kill

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>