i965/fs: Switch shader_time writes to using GRFs.

i965/fs: Switch shader_time writes to using GRFs.

This avoids conflicts between shader_time and FB writes, so we can include
more of the program under our profiling.  This does mean hiding more of
the message setup from the optimizer, which doesn't have a way to handle
multi-reg sends from GRFs.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>