drm/ttm: Fix compile error when CONFIG_SHMEM is not set

drm/ttm: Fix compile error when CONFIG_SHMEM is not set

When CONFIG_SHMEM is not set, the following compiler error occurs:

  ld: vmlinux.o: in function `ttm_backup_backup_page':
  (.text+0x10363bc): undefined reference to `shmem_writeout'
  make[3]: *** [/work/build/trace/nobackup/linux.git/scripts/Makefile.vmlinux:91: vmlinux.unstripped] Error 1

This is due to the replacement of writepage and calling swap_writeout()
and shmem_writeout() directly.  The issue is that when CONFIG_SHMEM is
not defined, shmem_writeout() is also not defined.

The function ttm_backup_backup_page() called mapping->a_ops->writepage()
which was then changed to call shmem_writeout() directly.

Even before commit 84798514db50 ("mm: Remove swap_writepage() and
shmem_writepage()"), it didn't make sense to call anything other than
shmem_writeout() as the ttm_backup deals only with shmem folios.

Have DRM_TTM config option select SHMEM to guarantee that
shmem_writeout() is available.

Link: https://lore.kernel.org/all/20250602170500.48713a2b@gandalf.local.home/
Suggested-by: Hugh Dickins <hughd@google.com>
Fixes: 84798514db50 ("mm: Remove swap_writepage() and shmem_writepage()")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>