radeonsi: compact MRTs to save PS export memory space

If there are holes between color outputs (e.g. a shader exports MRT1, but not MRT0), we can remove the holes by moving higher MRTs lower.

The hardware will remap the MRTs to their correct locations if we remove holes in SPI_SHADER_COL_FORMAT but not CB_SHADER_MASK.

This is a performance optimization, but MRTs with holes are pretty rare, so there is most likely no effect on any app.