drm/i915: Setup EMR first on all gen2-4

Unify the appaerance of the gen2-4 irq postinstall hooks a little
bit by doing the EMR setup first on all the platforms.

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/20170818183705.27850-7-ville.syrjala@linux.intel.com