drm/amdkfd: reserve a fence slot while locking the BO

Looks like the KFD still needs this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 8abc1eb2

 ("drm/amdkfd: switch over to using drm_exec v3")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231020123306.43978-1-christian.koenig@amd.com