drm/i915: Unify the appearance of gen3/4 irq_postistall hooks

Do the irq_mask/enable_mask setup in the same way on gen3/4, and also
reorder the steps to make the code more uniform.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-9-ville.syrjala@linux.intel.com