drm/i915: Stop overwriting RING_IMR in rcs resume

We store the engine->imr mask and set up the RING_IMR register on
restarting the engine. We do not then want to overwrite it with
an incomplete mask later as we may then lose interrupts!

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190418132720.3716-1-chris@chris-wilson.co.uk