samples: modules: lvgl: transparency: Fix supported pixel format check

Found while building with clang and -Wtautological-compare:

samples/modules/lvgl/screen_transparency/src/main.c:47:45: error:
bitwise or with non-zero value always evaluates to true
[-Werror,-Wtautological-bitwise-compare]

if (!(display_caps.supported_pixel_formats | PIXEL_FORMAT_ARGB_8888)) {
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Tom Hughes <tomhughes@chromium.org>