drm/i915: Kill the 830 MI_OVERLAY_OFF workaround

Now that we're disabling L2 clock gating MI_OVERLAY_OFF actually works
on 830, so let's use it.

v2: Nuke the unused dev_priv variable

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222195222.9618-1-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>