drm/amdkfd: add function svm_migrate_successful_pages

drm/amdkfd: add function svm_migrate_successful_pages

to get migration pages. dst MIGRATE_PFN_VALID bit and src
MIGRATE_PFN_MIGRATE bit should always be set when migration success.

cpage includes src MIGRATE_PFN_MIGRATE bit set and MIGRATE_PFN_VALID
bit unset pages for both RAM and VRAM when memory is only allocated
without being populated before migration, those ram pages should be
counted as migrated pages and those vram pages should not be counted
as migrated pages. Here migration pages refer to how many vram pages
invloved. Current svm_migrate_unsuccessful_pages only covers the
unsuccessful case that source is on RAM.

So far, we only see two unsuccessful migration cases. Since we
can clearly identify successful migration cases through dst
MIGRATE_PFN_VALID bit and src MIGRATE_PFN_MIGRATE bit within this
prange, also eventually successful migration pages will be used,
so we can use function svm_migrate_successful_pages to replace
function svm_migrate_unsuccessful_pages.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Philip Yang<Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>