drm/fb-helper: Cleanup checkpatch warnings

Fix up a couple of checkpatch warnings, such as whitespace or coding
style issues.

Tested-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170329144401.1804-2-thierry.reding@gmail.com