drm/i915: align vlv forcewake with common lore

For some odd reasons, the vlv forcewake code is rather different from
all other platforms, with no clear justification. Adjust things:

- Don't check whether the gt is awake already (and bail out early), we
  need to grab a forcewake anyway. Otherwise the chip might go to
  sleep too early. And this would also screw up our forcewake
  accounting.
- Like all other platforms, check whether the gt has cleared the
  forcewake bit in the _ACK register before setting it again.
- Use _MASKED_BIT_ENABLE/DISABLE macros
- Only use bit0 of the forcewake reg, not all 16 bits.
- check the gtfifodb reg like on all other platforms in _put.
- Drop the POSTING_READs for consistency.

v2: Failure to git add ... again.

v3: Fixup the spelling fail a bit.

Tested-by: "Purushothaman, Vijay A" <vijay.a.purushothaman@intel.com>
Tested-by: "Widawsky, Benjamin" <benjamin.widawsky@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>