drm/i915/ttm: Reorganize the ttm move code somewhat

In order to make the code a bit more readable and to facilitate
async memcpy moves, reorganize the move code a little. Determine
at an early stage whether to copy or to clear.

v2:
- Don't set up the memcpy iterators unless we are actually going to memcpy.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/20210813144331.372957-2-thomas.hellstrom@linux.intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20210813144331.372957-2-thomas.hellstrom@linux.intel.com