drm/i915/gem: Use local pointer ttm for __i915_ttm_move

Using local pointer ttm as argument in  __i915_ttm_move instead of bo->ttm,
as local pointer was previously assigned to bo->ttm in function.
This will make code a bit more readable.

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Jasmine Newsome <jasmine.newsome@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220104203642.231878-1-jasmine.newsome@intel.com