i965/fs: Make the repclear shader support either a uniform or a flat input

i965/fs: Make the repclear shader support either a uniform or a flat input

In the Vulkan driver we use a single flat input instead of a uniform
because setting up push constants is more disruptive to the pipeline than
setting up another vertex input.  This uses the number of uniforms as a key
to keep it working for the GL driver.

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