drm/i915: Create GEN specific read MMIO

Extracting the MMIO read functionality makes per gen handling a bit
simpler, and the overall function a lot easier to read. The increasing
complexity of reads doesn't get too bad as the generation number
increases:

gen[2-4]: Nothing special
gen5: ILK dummy write workaround
gen6+: forcewake shenanigans

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>