drm/i915: vlv: factor out valleyview_display_irq_install

We'll need to disable/re-enable the display-side IRQs when turning
off/on the VLV display power well. Factor out the helper functions
for this. For now keep the display IRQs enabled by default, so the
functionality doesn't change. This will be changed to enable/disable
the IRQs on-demand when adding support for VLV power wells in an
upcoming patch.

v2:
- take the irq spin lock for the whole enable/disable sequence as
  these can be called with interrupts enabled

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>