drm: fb_helper: Simplify exit condition

mode is always NULL at this point in the function, so make our intention
clear.

Signed-off-by: Daniel Stone <daniels@collabora.com>
[danvet: Stop clearing mode too to enlist gcc in tracking
uninitialized usage. And remove a space while at it.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>