drm/atomic: fix an error code in mode_fixup()

drm/atomic: fix an error code in mode_fixup()

Having "ret" be a bool type works for everything except
ret = funcs->atomic_check().  The other functions all return zero on
error but ->atomic_check() returns negative error codes.  We want to
propagate the error code but instead we return 1.

I found this bug with static analysis and I don't know if it affects
run time.

Fixes: 3fd24a8ab259 ("drm/atomic: Add ->atomic_check() to encoder helpers")
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170207234601.GA23981@mwanda