modules: lvgl: Force fullsize render buffer on full refresh

Changes the default of `LV_Z_VDB_SIZE` to 100 percent if
`LV_Z_FULL_REFRESH` is set. Reason is that LVGL will reset the full
refresh flag if the buffer is not equal to the screen size.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>