drm/amdgpu: Fix the incomplete product number

The comments say that the product number is a 16-digit HEX string so the
buffer needs to be at least 17 characters to hold the NUL terminator. Expand
the buffer size to 20 to avoid the alignment issues.

The comment:Product number should only be 16 characters. Any
more,and something could be wrong. Cap it at 16 to be safe

Signed-off-by: Roy Sun <Roy.Sun@amd.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>