drm/amdgpu: init return value in amdgpu_ttm_clear_buffer

Otherwise an uninitialized value can be returned if
amdgpu_res_cleared returns true for all regions.

Possibly closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3812

Fixes: a68c7eaa7a8f ("drm/amdgpu: Enable clear page functionality")
Signed-off-by: Pierre-Eric Pelloux-Prayer <[e-mail]>
Acked-by: Alex Deucher <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
(cherry picked from commit 7c62aacc3b452f73a1284198c81551035fac6d71)
Cc: [e-mail]

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

···
 	struct amdgpu_ring *ring = adev->mman.buffer_funcs_ring;
 	struct amdgpu_res_cursor cursor;
 	u64 addr;
-	int r;
+	int r = 0;
 
 	if (!adev->mman.buffer_funcs_enabled)
 		return -EINVAL;