i965/fs: Use offset a lot more places

i965/fs: Use offset a lot more places

We have this wonderful offset() function for advancing registers, but we're
not using it.  Using offset() allows us to do some sanity checking and
avoid manually touching fs_reg::reg_offset.  In a few commits, we will make
offset do even more nifty things for us.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>