drm/i915: Unmask interrupt for render engine on Sandybridge

With splitted engines on Sandybridge, each engine has its own
interrupt control as well. This unmasks the interrupt to properly
enable pipe control notify event for render engine.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>