drm/amdgpu: Fix out-of-bound access for gfx_v10_0_ring_test_ib()

The gfx_v10_0_ring_test_ib() function uses 20 bytes instead of 16
bytes during the test. The patch sets the size of the allocation to be
4-byte larger to match the actual usage.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Haohui Mai <ricetons@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>