i965/gs: Allow src0 immediates in GS_OPCODE_SET_WRITE_OFFSET.

i965/gs: Allow src0 immediates in GS_OPCODE_SET_WRITE_OFFSET.

GS_OPCODE_SET_WRITE_OFFSET is a MUL with a constant src[1] and special
strides.  We can easily make the generator handle constant src[0]
arguments by instead generating a MOV with the product of both operands.

This isn't necessarily a win in and of itself - instead of a MUL, we
generate a MOV, which should be basically the same cost.  However, we
can probably avoid the earlier MOV to put src[0] into a register.

shader-db statistics for geometry shaders only:

total instructions in shared programs: 3207 -> 3173 (-1.06%)
instructions in affected programs:     3207 -> 3173 (-1.06%)
helped:                                11

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>