drm/aspeed: Fix Kconfig warning & subsequent build errors

Kernel test robot reported build errors (undefined references)
that didn't make much sense. After reproducing them, there is also
a Kconfig warning that is the root cause of the build errors, so
fix that Kconfig problem.

Fixes this Kconfig warning:
WARNING: unmet direct dependencies detected for CMA
  Depends on [n]: MMU [=n]
  Selected by [m]:
  - DRM_ASPEED_GFX [=m] && HAS_IOMEM [=y] && DRM [=m] && OF [=y] && (COMPILE_TEST [=y] || ARCH_ASPEED) && HAVE_DMA_CONTIGUOUS [=y]

and these dependent build errors:
(.text+0x10c8c): undefined reference to `start_isolate_page_range'
microblaze-linux-ld: (.text+0x10f14): undefined reference to `test_pages_isolated'
microblaze-linux-ld: (.text+0x10fd0): undefined reference to `undo_isolate_page_range'

Fixes: 76356a9 ("drm: aspeed: Clean up Kconfig options")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Andrew Jeffery <andrew@aj.id.au>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: linux-mm@kvack.org
Cc: linux-aspeed@lists.ozlabs.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20201011230131.4922-1-rdunlap@infradead.org
Signed-off-by: Joel Stanley <joel@jms.id.au>

drivers/gpu/drm/aspeed/Kconfig

@@ -3,6 +3,7 @@ config DRM_ASPEED_GFX
 	tristate "ASPEED BMC Display Controller"
 	depends on DRM && OF
 	depends on (COMPILE_TEST || ARCH_ASPEED)
+	depends on MMU
 	select DRM_KMS_HELPER
 	select DRM_KMS_CMA_HELPER
 	select DMA_CMA if HAVE_DMA_CONTIGUOUS