drm/i915: Reorder phys backing storage release

drm/i915: Reorder phys backing storage release

In commit a4f5ea64f0a8 ("drm/i915: Refactor object page API"), I
reordered the object->pages teardown to be more friendly wrt to a
separate obj->mm.lock. However, I overlooked the phys object and left it
with a dangling use-after-free of its phys_handle. Move the allocation
of the phys handle to get_pages and it release to put_pages to prevent
the invalid access and to improve symmetry.

v2: Add commentary about always aligning to page size.

Testcase: igt/drv_selftest/objects
Reported-by: Ville Syrjälä <[email protected]>
Fixes: a4f5ea64f0a8 ("drm/i915: Refactor object page API")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit dbb4351bab0a8440f6b02895c142bce6c30b7097)
Signed-off-by: Jani Nikula <[email protected]>