r600: increase performance for DRI PRIME offloading if 2nd GPU is Evergreen+

This is a direct port of Marek Olšáks patch
"radeonsi: increase performance for DRI PRIME
offloading if 2nd GPU is CIK or VI" to r600.

It uses SDMA for the detiling blit from renderoffload VRAM
to GTT, as SDMA is much faster for tiled->linear blits from
VRAM to GTT.

Testing on a dual Radeon HD-5770 setup reduced the time
for the render offload gpu to get its rendering into
system RAM from approximately 16 msecs for simple rendering
at 1920x1080 pixel 32 bpp to 5 msecs, a > 3x speedup!

This was measured using ftrace to trace the time the radeon kms
driver waited on the dmabuf fence of the renderoffload gpu to
complete.

All in all this brought the time for a flip down from 20 msecs
to 9 msecs, so the prime setup can display at full 60 fps instead
of barely 30 fps vsync'ed.

The current r600 implementation supports SDMA on Evergreen and
later, but not R600/R700 due to some bugs apparently present
in their SDMA implementation.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>

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