drm/i915: Don't mix srcu tag and negative error codes

While srcu may use an integer tag, it does not exclude potential error
codes and so may overlap with our own use of -EINTR. Use a separate
outparam to store the tag, and report the error code separately.

Fixes: 2caffbf1 ("drm/i915: Revoke mmaps and prevent access to fence registers across reset")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190912160834.30601-1-chris@chris-wilson.co.uk