drm/gpuva_mgr: remove unused prev pointer in __drm_gpuva_sm_map()

The prev pointer in __drm_gpuva_sm_map() was used to implement automatic
merging of mappings. Since automatic merging did not make its way
upstream, remove this leftover.

Fixes: e6303f32

 ("drm: manager to keep track of GPUs VA mappings")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230823233119.2891-1-dakr@redhat.com