drm/i915: Move DPINVGTT setup to vlv_display_irq_reset()

DPINVGTT lives inside the disp2d power well so we can't frob it unless
we know the power well is active. Let's this stuff into
vlv_display_irq_reset() which is only called at the right times so that
we don't get unclaimed register access errors.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94164


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460382992-28728-10-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>