iris: Skip allocating a null surface when there are 0 color regions.

iris: Skip allocating a null surface when there are 0 color regions.

The compiler now sets the "Null Render Target" bit in the RT write
extended message descriptor, causing it to write to an implicit null
surface without us needing to set one up in the binding table.

Together with the last patch, this improves performance in Car Chase on
an Icelake 8x8 (locked to 700Mhz) by 0.0445526% +/- 0.0132736% (n=832).

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>