drm/fb-helper: Fix potential NULL pointer dereference

fb_helper is being dereferenced before it is null checked,
hence there is a potential null pointer dereference.

Fix this by moving the pointer dereference after fb_helper
has been null checked.

This issue was detected with the help of Coccinelle.

Fixes: c777990f ("drm/fb-helper: Handle function NULL argument")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171205174628.GA31189@embeddedor.com