r600g: improve bo flushing

r600g: improve bo flushing

Flush read cache before writting register. Track flushing inside
of a same cs and avoid reflushing same bo if not necessary. Allmost
properly force flush if bo rendered too and then use as a texture
in same cs (missing pipeline flush dunno if it's needed or not).

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