[Intel-gfx] Write GFX_FLSH_CNT after updating GGTT entries

[Zhi Wang]

Hi Gurus:
     I'm curious about the register GFX_FLSH_CNT(0x101008) in 
i915_gem_gtt.c. Does these register exist in recently generations? After 
digging into b-spec, it looks only BXT and CHV has this register. Does 
the desktop platform also have this register which needs to be written 
after updating GGTT MMIOs?

BTW: Looks windows driver haven't used this MMIO... So whose behavior is 
the right behavior?

Thanks,
Zhi.