drm/hisilicon: Add depends on MMU

The Kconfig symbol depended on MMU but was dropped by the commit
acad3fe6 ("drm/hisilicon: Removed the dependency on the mmu")
because it already had as a dependency ARM64 that already selects MMU.

But later, commit a0f25a6b ("drm/hisilicon/hibmc: Allow to be built
if COMPILE_TEST is enabled") allowed the driver to be built for non-ARM64
when COMPILE_TEST is set but that could lead to unmet direct dependencies
and linking errors.

Prevent a kconfig warning when MMU is not enabled by making
DRM_HISI_HIBMC depend on MMU.

WARNING: unmet direct dependencies detected for DRM_TTM
  Depends on [n]: HAS_IOMEM [=y] && DRM [=m] && MMU [=n]
  Selected by [m]:
  - DRM_TTM_HELPER [=m] && HAS_IOMEM [=y] && DRM [=m]
  - DRM_HISI_HIBMC [=m] && HAS_IOMEM [=y] && DRM [=m] && PCI [=y] && (ARM64 || COMPILE_TEST [=y])

Fixes: acad3fe6 ("drm/hisilicon: Removed the dependency on the mmu")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Xinliang Liu <xinliang.liu@linaro.org>
Cc: Tian Tao  <tiantao6@hisilicon.com>
Cc: John Stultz <jstultz@google.com>
Cc: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Cc: Chen Feng <puck.chen@hisilicon.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220531025557.29593-1-rdunlap@infradead.org