r600g: move streamout state to drivers/radeon

r600g: move streamout state to drivers/radeon

This streamout state code will be used by radeonsi.

There are new structures r600_common_context and r600_common_screen.
What is inherited by what is shown here:

pipe_context -> r600_common_context -> r600_context
pipe_screen -> r600_common_screen -> r600_screen

The common structures reside in drivers/radeon. Currently they only contain
enough functionality to be able to handle streamout. Eventually I'd like
the whole pipe_screen implementation to be shared and some of the context
stuff too.

This is quite big, but most changes are because of the new structures and
the fact r600_write_value is replaced by radeon_emit.

Thanks to Tom Stellard for fixing the build for r600g/compute.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Tom Stellard <thomas.stellard@amd.com>