drm/amdkfd: clarify the origin of cpages returned by migration functions

cpages is only updated by migrate_vma_setup. So capture its value at
that point to clarify the significance of the number. The next patch
will add counting of actually migrated pages after migrate_vma_pages for
debug purposes.

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