drm/pagemap Fix error paths in drm_pagemap_migrate_to_devmem

drm/pagemap Fix error paths in drm_pagemap_migrate_to_devmem

Avoid unlocking and putting device pages unless they were successfully
locked, and do not calculate migrated_pages on error paths.

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: 75af93b3f5d0 ("drm/pagemap, drm/xe: Support destination migration over interconnect")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patch.msgid.link/20260107182716.2236607-2-matthew.brost@intel.com