drm/amdgpu/mes: fix use-after-free issue

Delete fence fallback timer to fix the ramdom
use-after-free issue.

v2: move to amdgpu_mes.c

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>