drm/i915: proper use of forcewake

Moved the macros around to properly do reads and writes for the given
GPU. This is to address special requirements for gen6 (SNB) reads and
writes.

Registers in the range 0-0x40000 on gen6 platforms require special
handling. Instead of relying on the callers to pick the registers
correctly, move the logic into the read and write functions.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>