drm/hisilicon: Removed the dependency on the mmu

hibmc can only be used in ARM64 architectures, and mmu defaults to y
in arch/arm64/Kconfig, so there is no need to add a dependency on mmu
in hibmc's kconfig.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1598428528-49046-1-git-send-email-tiantao6@hisilicon.com