drm/ttm: fix error handling if no BO can be swapped out v4

In case that all pre-allocated BOs are busy, just continue to populate
BOs since likely half of system memory in total is still free.

v4 (chk): fix code moved to VMWGFX as well

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210422115757.3946-1-christian.koenig@amd.com