drm/i915/ttm: Move the i915_gem_obj_copy_ttm() function

Move the i915_gem_obj_copy_ttm() function to i915_gem_ttm_move.h.
This will help keep a number of functions static when introducing
async moves.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211122214554.371864-3-thomas.hellstrom@linux.intel.com