lvgl: take opacity into account

The callback used for the 32-bit color depth has a slight problem, it
doesn't take opacity into account. Correctly mix the colors by using
lv_color_mix().

Suggested-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@huawei.com>