radeonsi: Set SPI_SHADER_COL_FORMAT to what the pixel shader actually exports.

radeonsi: Set SPI_SHADER_COL_FORMAT to what the pixel shader actually exports.

Instead of deriving it from the colour buffer formats only.

Fixes a number of piglit tests which export depth from the pixel shader.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>