modules: lvgl: replace LV_Z_DPI with LV_DPI_DEF

It looks like LV_Z_DPI is not used anywhere and only lived in the
context of an incorrect board symbol definition so far (moved to
Kconfig.lvgl in the previous patch).

This was probably meant to be LV_DPI_DEF (which actually exists in the
LVGL module) and was renamed incorrectly, fix it by renaming it
everywhere.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>