drm/amdgpu: fix memory overflow in the IB test

Fix a memory overflow issue in the gfx IB test
for some ASICs. At least 20 bytes are needed for
the IB test packet.

v2: correct code indentation errors. (Christian)

Signed-off-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org