drm/i915: Extract knowledge of register forcewake domains

Knowledge of which register per platform belonds in which
forcewake domain was embedded in the MMIO accessors themselves.

Extract it into standalone macros so they can be used from
new code in the following patches.

This causes GCC to compile some of the MMIO accessors slightly
differently and grows the code a tiny amount. But none of the
growth is on the fast-path so it does not matter hugely.

Affected sizes before:

00000000000026f0 00000000000001a5 t gen6_read16
0000000000002390 00000000000001a5 t gen6_read32
00000000000028a0 00000000000001a5 t gen6_read64

00000000000061d0 000000000000019e t gen8_write16
0000000000006510 000000000000019d t gen8_write32
0000000000006370 000000000000019d t gen8_write64
00000000000021f0 000000000000019d t gen8_write8

Affected sizes after:

0000000000002840 00000000000001aa t gen6_read16
00000000000024e0 00000000000001a9 t gen6_read32
00000000000029f0 00000000000001a9 t gen6_read64

0000000000004f20 00000000000001b5 t gen8_write16
0000000000004ba0 00000000000001b4 t gen8_write32
00000000000050e0 00000000000001b4 t gen8_write64
0000000000004d60 00000000000001b4 t gen8_write8

Other MMIO accessors are not affected in size.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>