drm/ttm: Import Linux commit 5e45d7dfd74100d622f9cdc70bfd1f9fae1671de

drm/ttm: Import Linux commit 5e45d7dfd74100d622f9cdc70bfd1f9fae1671de

Author: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Date:   Tue Jan 15 14:57:05 2013 +0100

    drm/ttm: add ttm_bo_reserve_slowpath

    Instead of dropping everything, waiting for the bo to be unreserved
    and trying over, a better strategy would be to do a blocking wait.

    This can be mapped a lot better to a mutex_lock-like call.

    Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
    Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Approved by:	kib@