drm/amdkfd: Fix the case that vm range is hole at svm_migrate_copy_to_vram

In the Linux kernel, the following vulnerability has been resolved: drm/amdkfd: Fix the case that vm range is hole at svm_migrate_copy_to_vram When migration vm range is hole at cpu side(MIGRATE_PFN_MIGRATE set + MIGRATE_PFN_VALID unset) driver still allocates device pages. There is no dma map of src pages and migration. j is 0 and svm_migrate_copy_memory_gart() will return an uninitialized r. That can trigger out_free_vram_pages to drop all VRAM just set up. Initialize r and only call the last svm_migrate_copy_memory_gart if j > 0. Current code postponed the last page to the final copy. This patch flushes on the last page when reach to the end of current drm_buddy_block; avoids another svm_migrate_copy_memory_gart.