drm/i915: add i915_gem_object_create_region_at()

Add a generic interface for allocating an object at some specific
offset, and convert stolen over. Later we will want to hook this up to
different backends.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315181425.576828-4-matthew.auld@intel.com