drm/i915: Clear GDSR after reset on ILK

Clear the reset domain after a succesful GPU reset on ilk. We already
do that on gen4, so let's try to be a bit more consistent. And if
ether render or media reset fails, we might use the leftover value
in the register to pinpoint the culprit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>