drm/i915: Fix some NULL vs IS_ERR() conditions

There were several places which check for NULL when they should have
been checking for IS_ERR().

Fixes: d8af05ff ("drm/i915: Allow sharing the idle-barrier from other kernel requests")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807122832.GA10517@mwanda