drm/i915: properly reenable gen8 pipe IRQs

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: [email protected]
Signed-off-by: Paulo Zanoni <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>