drm/panel: st7703: Use dev_err_probe

Less code and easier probe deferral debugging.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/36405038d93eb148f3e8ed8e5ea70de8e87afd78.1605688147.git.agx@sigxcpu.org