drm/fb-helper: Improve code readability

Add a couple of temporary variables and use shorter names for existing
variables in drm_fb_helper_add_one_connector() for better readability.

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/20170329144401.1804-4-thierry.reding@gmail.com