drm/tiny: pixpaper: add explicit dependency on MMU

The DRM_GEM_SHMEM_HELPER helper requires MMU enabled because it uses
vmf_insert_pfn() in its mmap implementation. On NOMMU configurations
(e.g. some RISC-V randconfig builds), this symbol is unavailable and
selecting DRM_GEM_SHMEM_HELPER causes a modpost undefined reference:

ERROR: modpost: "vmf_insert_pfn" [drivers/gpu/drm/drm_shmem_helper.ko] undefined!

Normally, Kconfig prevents this helper from being selected when
CONFIG_MMU=n. However, in some randconfig builds (such as those used by
0day CI), select statements can override unmet dependencies, triggering
the issue.

Add an explicit dependency on MMU to DRM_PIXPAPER to prevent this.

Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Fixes: 0c4932f6ddf8 ("drm/tiny: pixpaper: Fix missing dependency on DRM_GEM_SHMEM_HELPER")
Acked-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: LiangCheng Wang <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

drivers/gpu/drm/tiny/Kconfig

···
 config DRM_PIXPAPER
         tristate "DRM support for PIXPAPER display panels"
         depends on DRM && SPI
+        depends on MMU
         select DRM_CLIENT_SELECTION
         select DRM_GEM_SHMEM_HELPER
         select DRM_KMS_HELPER