i965: Switch on shader stage in nir_setup_outputs().

i965: Switch on shader stage in nir_setup_outputs().

Adding new shader stages to a switch statement is less confusing than an
if-else-if ladder where all but the first case are fragment shader
specific (but don't claim to be).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>