[Mesa-dev] r600g async dma support

[j.glisse at gmail.com]

So design is mostly the same then previously. Few changes, first i use only
one thread to offload all cs submission wether gfx or dma. Reasons is that
using on thread for gfx and one for dma lead to more complex synchronization
with no gain ie when submitting gfx you would need to make sure previous
dma submittion are done and vice et versa. So in the end it's just not a
good idea. Moreover the dma submission is lot faster than the gfx one as
the dma cs are smaller and simpler to parse for the kernel.

Second is that i don't use a stack in r600g to keep track of cs submission
ordering. Instead anytime r600g switch cmd stream ie start writing dma
command after writing gfx one, we first asynchronously flush the gfx
command. This insure that any point in time the driver is only building
command for either gfx or dma ring and everything is serialize from driver
pov. It simplify implementation as there is no need to special case some
corner case such as query/event or streamout buffer.

The last patch is a small optimization that decrease the cpu overhead by
not submitting gfx cmd that does not do anything.

Everything been tested on r7xx and evergreen and i witnessed no regression.

Evergreen can be improved by adding support for partial blit but i am not
sure it's worth it.

Cheers,
Jerome