drm/i915: set "ret" correctly on error paths

If "crtc" is NULL, then my static checker complains that "ret" isn't
initialized on that path.  It doesn't really cause a problem unless
"ret" is somehow set to -EDEADLK which is not likely.

Chris Wilson also noticed another error path where "ret" isn't set
correctly.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170414195425.GA8144@mwanda


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>