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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510280213.0rlYA4T3-lkp@intel.com/
Fixes: 0c4932f ("drm/tiny: pixpaper: Fix missing dependency on DRM_GEM_SHMEM_HELPER")
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: LiangCheng Wang <zaq14760@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20251028-bar-v1-1-edfbd13fafff@gmail.com

Author: lc-wang

drivers/gpu/drm/tiny/Kconfig

@@ -85,6 +85,7 @@ config DRM_PANEL_MIPI_DBI
 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