drm/ttm: do not keep GPU dependent addresses

GPU address handling is device specific and should be handle by its device
driver.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/372937/


Signed-off-by: Christian König <christian.koenig@amd.com>