drm/i915: switch order of power domain init wrt. irq install

On VLV at least the display IRQ register access and functionality
depends on its power well to be on, so move the power domain HW init
before we install the IRQs.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>