Fix FragColorExport pass
Fix FragColorExport pass

Fix two issues in FragColorExport pass:
* SPI_SHADER_COL_FORMAT was not updated in some cases,
  due to an early exit in addExportForGenericOutput()
  causing updateSpiShaderColFormat() to never be called
  in some shaders.
* A check for dummy export was lost leading to generating
  unnecessary dummy exports on gfx10.

Fixes: #774

googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc,googlestadia/llpc