drm/amdgpu: silence UBSAN warning

[ Upstream commit 05d9e24d ]

Convert a variable sized array from [1] to [].

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>