drm/i915: Unbreak interrupts on pre-gen6

Prior to gen6 we didn't have per-ring IMR registers, which means that
since commit 61ff75ac ("drm/i915: Simplify enabling
user-interrupts with L3-remapping") we're now masking off all interrupts
when init_render_ring() gets called. That's rather rude. Let's limit
the ring IMR frobbing to machines that actually have the per-ring IMR
registers.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 61ff75ac

 ("drm/i915: Simplify enabling user-interrupts with L3-remapping")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468340687-3596-1-git-send-email-ville.syrjala@linux.intel.com
Reviewd-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit 035ea405

)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>