drm/i915: Create GEN specific write MMIO

Similar to the previous patch which implemented GEN specific reads; this
patch does the same for writes. Writes have a bit of adding complexity
due to the FPGA_DBG feature of HSW plus:

gen[2-4]: nothing special
gen5: ILK dummy write
gen[6-7]: forcewake shenanigans
gen[HSW}: forcewake shenanigans + FPGA_DBG

I was a bit torn about whether or not to combine 6-HSW as one function,
since the FPGA_DBG is cleanly separated, and it wouldn't make the 6-7
MMIO too messy. In the end, I chose the clearest possible solution which
splits out HSW.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>