drm/i915/gem: Break out some shmem backend utils

Break out some shmem backend utils for future reuse by the TTM backend:
shmem_alloc_st(), shmem_free_st() and __shmem_writeback() which we can
use to provide a shmem-backed TTM page pool for cached-only TTM
buffer objects.

Main functional change here is that we now compute the page sizes using
the dma segments rather than using the physical page address segments.

v2(Reported-by: kernel test robot <[email protected]>)
    - Make sure we initialise the mapping on the error path in
      shmem_get_pages()

Signed-off-by: Thomas Hellström <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Signed-off-by: Matthew Auld <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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