i965/blorp: Cleanup leftovers from push constant disabling

i965/blorp: Cleanup leftovers from push constant disabling

Setup for pixel shader push constants is the same as for other
stages. Note that on gen8+ the if-else branches were identical
and the generation check for packet size redundant.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>