radeonsi/gfx10: add si_context::emit_cache_flush

radeonsi/gfx10: add si_context::emit_cache_flush

The introduction of GCR_CNTL makes cache flush handling on gfx10
sufficiently different that it makes sense to just use a separate
function.

Since emit_cache_flush is called quite early during context init,
we initialize the pointer explicitly in si_create_context.

Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>