drm/amdgpu: create fence slab once when amdgpu module init.

This avoids problems with multiple GPUs.  For example,
if the first GPU failed before amdgpu_fence_init() was
called, amdgpu_fence_slab_ref is still 0 and it will
get decremented in amdgpu_fence_driver_fini().  This
will lead to a crash during init of the second GPU since
amdgpu_fence_slab_ref is not 0.

v2: add functions for init/exit instead of
    moving the variables into the driver.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu.h b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
index 2a009c3..992f00b 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
@@ -602,6 +602,8 @@
 void amdgpu_sync_free(struct amdgpu_sync *sync);
 int amdgpu_sync_init(void);
 void amdgpu_sync_fini(void);
+int amdgpu_fence_slab_init(void);
+void amdgpu_fence_slab_fini(void);
 
 /*
  * GART structures, functions & helpers