samples: display: lvgl: increase main stack size to 4k

When tested with xiao_esp32s3 + seeed_xiao_round_display main stack is
occupied with 3504 bytes (checked with 'kernel thread stacks' command).

Increase main stack size from 2k to 4k in order to make this sample work at
runtime.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>