drm/ttm: Remove unnecessary '0' values from ret

The variable ret is assigned in the judgment branch statement, he does
not need to initialize the assignment.

Signed-off-by: Li zeming <zeming@nfschina.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907032934.4490-1-zeming@nfschina.com


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