i965/eu: Define alternative interface for setting compression and group controls.

This implements some simple helper functions that can be used to
specify the group of channel enable signals and compression enable
that apply to a brw_inst instruction.

It's intended to replace brw_set_default_compression_control
eventually because the current interface has a number of shortcomings
inherited from the Gen-4-5-centric representation of compression and
group controls as a single non-orthogonal enum: On the one hand it
doesn't work for specifying arbitrary group controls other than 1Q and
2Q, which are frequently useful in SIMD32 and FP64 programs.  On the
other hand the current interface forces you to update the compression
*and* group controls simultaneously, which has been the source of a
number of generator bugs (a bunch of them fixed in this series),
because in many cases we would end up resetting the group controls to
zero inadvertently even though everything we wanted to do was disable
instruction compression -- The latter seems especially unfortunate on
Gen6+ hardware which have no explicit compression control, so we would
end up bashing the quarter control field of the instruction for no
benefit.

Instead of a single function that updates both at the same time
introduce separate interfaces to update one or the other independently
preserving the current value of the other (which typically comes from
the back-end IR so it has to be respected).

Reviewed-by: Jason Ekstrand <[email protected]>

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