drm/i915/gem: Add a helper to read data from a GEM object page

Add a simple helper to read data with the CPU from the page of a GEM
object. Do the read either via a kmap if the object has struct pages
or an iomap otherwise. This is needed by the next patch, reading a u64
value from the object (w/o requiring the obj to be mapped to the GPU).

Suggested by Chris.

v2 (Chris):
- Sanitize the type and order of func params.
- Avoid consts requiring too many casts.
- Use BUG_ON instead of WARN_ON, simplify the conditions.
- Fix __iomem sparse errors.
- Leave locking/syncing/pinning up to the caller, require only that the
  caller has pinned the object pages.
- Check for iomem backing store before reading via an iomap.
v3:
- Fix offset passed to io_mapping_map_wc() missing a mem.region.start
  delta. (Chris, Matthew)

Cc: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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