drm/i915/skl: Gen9 multi-engine forcewake

Enable multi-engine forcewake for Gen9.

v2: (Damien)
- Rebase on top of nightly
- Move the register range definitions to intel_uncore.c
- Whitespace fixes

v3: (Addressing Mika's comment, done by Damien)
- Use REG_RANGE() (introduced after the patch was written)
- Add a SKL_NEEDS_FORCE_WAKE() macro that gets rid of a useless
  comparison to FORCEWAKE (reg 0xa18c is not used on SKL)

v4: (Damien)
- Use newly introduced ASSIGN_READ/WRITE_MMIO_VFUNCS() macros

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Zhe Wang <zhe1.wang@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>