drm/i915: fix Haswell pfit power well check v2

We can't read the pfit regs if the power well is off, so use the cached
value.

v2: re-add lost comment (Jesse)
    make sure the crtc using the fitter is actually enabled (Jesse)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Drop now unused dev_priv, as spotted by Mika.]
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>