drm/i915: Reinstate GMBUS and AUX interrupts on gen4/g4x

drm/i915: Reinstate GMBUS and AUX interrupts on gen4/g4x

Now that we're not using MSI anymore on gen4 we can start
using GMBUS and AUX interrupts again. These were disabled on
account of them causing the hardware to somehow generate
legacy interrupts even when MSI was enabled.

See commit c12aba5aa0e6 ("drm/i915: stop using GMBUS IRQs on Gen4
chips") and commit 4e6b788c3f23 ("drm/i915: Disable dp aux irq on
g4x") for more details.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-17-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>