i965: Use a single index per shader for shader_time.

i965: Use a single index per shader for shader_time.

Previously, each shader took 3 shader time indices which were potentially
at arbirary points in the shader time buffer.  Now, each shader gets a
single index which refers to 3 consecutive locations in the buffer.  This
simplifies some of the logic at the cost of having a magic 3 a few places.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>