drm/i915: properly reenable gen8 pipe IRQs

We were missing the pipe B/C vblank bits! Take a look at
gen8_de_irq_postinstall for a comparison.

This should fix a bunch of IGT tests.

There are a few more things we could improve on this code, but this
should be the minimal fix to unblock us.

v2: s/extra_iir/extra_ier/ because IIR doesn't make sense (Ville)

Bugzilla:https://bugs.freedesktop.org/show_bug.cgi?id=83640


Testcase: igt/*
Cc: stable@vger.kernel.org
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>