[intel-gem] Only update obj->write_domain if we're actually changing it.

The problem was revealed where on 965, the display list vertex buffer would see:

create		      -> (CPU, CPU)
set_domain (CPU, CPU) -> (CPU, CPU)
set_comain (CPU, 0)   -> (CPU, 0) (no clflush occurred)
execbuf	   (GPU, 0)   -> (CPU+GPU, 0) (still no clflush)

instead of:

create		      -> (CPU, CPU)
set_domain (CPU, CPU) -> (CPU, CPU)
set_comain (CPU, 0)   -> (CPU, CPU)
execbuf	   (GPU, 0)   -> (CPU+GPU, 0) (clflushed)