sna/gen6: Prefer the BLT ring, except for copies on behalf of DRI

As demonstrated by the all-important trap300, using the BLT is 2x faster
than the RENDER ring for the simple case of solid fills. (Though note
that performing the relocations costs 3x as much CPU for 2x GPU
performance.) One case that may regress from this change is copywinpix
which should benefit from the batching in the RENDER commands, and might
warrant revisiting in the future (with realistic and synthetic
benchmarks in hand!)

However, due to the forced stall when switching rings, we still want to
perform RENDER copies on behalf of DRI clients and before page-flips.

Checking against cairo-perf-trace indicated no major impact -- I had
worried that setting the BLT flag for some clears might have had a
knock-on effect causing too many operations that could be pipelined on
the RENDER ring to be sent to the BLT ring instead.

Reported-by: Michael Larabel <Michael@phoronix.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

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