drm/i915: Fix GT wake FIFO free entries for VLV

On VLV the GTFIFOCTL register has other bits besides the number of free
entries in the GT wake FIFO. Apply a mask when we read th register to
make sure we don't misinterpret the number of free FIFO entries.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: There's some unclarity about hsw, but brushed off as todays'
Bspec just acting up a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>