drm/ttm: make eviction decision a driver callback v2

This way the driver can decide if it is valuable to evict a BO or not.

The current implementation is added as default to all existing drivers.

v2: fix some typos found during internal testing

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>