r600g: rework flusing and synchronization pattern v7

r600g: rework flusing and synchronization pattern v7

This bring r600g allmost inline with closed source driver when
it comes to flushing and synchronization pattern.

v2-v4: history lost somewhere in outer space
v5: Fix compute size of flushing, use define for flags, update
    worst case cs size requirement for flush, treat rs780 and
    newer as r7xx when it comes to streamout.
v6: Fix num dw computation for framebuffer state, remove dead
    code, use define instead of hardcoded value.
v7: Remove dead code

Signed-off-by: Jerome Glisse <jglisse@redhat.com>