drm/lima: add missing Kconfig dependency

Current implementation does not support MMU-less
plarforms.

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Fixes: a1d2a633 ("drm/lima: driver for ARM Mali4xx GPUs")
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190409003617.4470-1-yuq825@gmail.com