drm/i915: Use kmap_local_page() in gem/i915_gem_object.c

The use of kmap_atomic() is being deprecated in favor of
kmap_local_page()[1], and this patch converts the call from
kmap_atomic() to kmap_local_page().

The main difference between atomic and local mappings is that local
mappings doesn't disable page faults or preemption (the preemption is
disabled for !PREEMPT_RT case, otherwise it only disables migration).

With kmap_local_page(), we can avoid the often unwanted side effect of
unnecessary page faults and preemption disables.

There're 2 reasons why i915_gem_object_read_from_page_kmap() doesn't
need to disable pagefaults and preemption for mapping:

1. The flush operation is safe. In drm/i915/gem/i915_gem_object.c,
i915_gem_object_read_from_page_kmap() calls drm_clflush_virt_range() to
use CLFLUSHOPT or WBINVD to flush. Since CLFLUSHOPT is global on x86
and WBINVD is called on each cpu in drm_clflush_virt_range(), the flush
operation is global.

2. Any context switch caused by preemption or page faults (page fault
may cause sleep) doesn't affect the validity of local mapping.

Therefore, i915_gem_object_read_from_page_kmap() is a function where
the use of kmap_local_page() in place of kmap_atomic() is correctly
suited.

Convert the calls of kmap_atomic() / kunmap_atomic() to
kmap_local_page() / kunmap_local().

And remove the redundant variable that stores the address of the mapped
page since kunmap_local() can accept any pointer within the page.

[1]: https://lore.kernel.org/all/[email protected]

Suggested-by: Dave Hansen <[email protected]>
Suggested-by: Ira Weiny <[email protected]>
Suggested-by: Fabio M. De Francesco <[email protected]>
Signed-off-by: Zhao Liu <[email protected]>
Reviewed-by: Ira Weiny <[email protected]>
Reviewed-by: Fabio M. De Francesco <[email protected]>
Signed-off-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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