drm/tiny: pixpaper: Fix missing dependency on DRM_GEM_SHMEM_HELPER

drm/tiny: pixpaper: Fix missing dependency on DRM_GEM_SHMEM_HELPER

The driver uses drm_gem_shmem_prime_import_no_map() and
drm_gem_shmem_dumb_create(), but the Kconfig currently selects
DRM_GEM_DMA_HELPER instead of DRM_GEM_SHMEM_HELPER. This causes
link failures when DRM_GEM_SHMEM_HELPER is not enabled.

Select DRM_GEM_SHMEM_HELPER to fix the build.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509220320.gfFZjmyg-lkp@intel.com/
Fixes: c9e70639f591 ("drm: tiny: Add support for Mayqueen Pixpaper e-ink panel")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: LiangCheng Wang <zaq14760@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250922-bar-v1-1-b2a1f54ace82@gmail.com