drm/xe/migrate: Fix alignment check

drm/xe/migrate: Fix alignment check

The check would fail if the address is unaligned, but not when
accounting the offset. Instead of `buf | offset` it should have
been `buf + offset`. To make it more readable and also drop the
uintptr_t, just use the IS_ALIGNED() macro.

Fixes: 270172f64b11 ("drm/xe: Update xe_ttm_access_memory to use GPU for non-visible access")
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/20250710-migrate-aligned-v1-1-44003ef3c078@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>