r600g: add cs memory usage accounting and limit it v3

r600g: add cs memory usage accounting and limit it v3

We are now seing cs that can go over the vram+gtt size to avoid
failing flush early cs that goes over 70% (gtt+vram) usage. 70%
is use to allow some fragmentation.

The idea is to compute a gross estimate of memory requirement of
each draw call. After each draw call, memory will be precisely
accounted. So the uncertainty is only on the current draw call.
In practice this gave very good estimate (+/- 10% of the target
memory limit).

v2: Remove left over from testing version, remove useless NULL
    checking. Improve commit message.
v3: Add comment to code on memory accounting precision

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>