drm/amdgpu: correct default gfx wdt timeout setting

When gfx wdt was configured to fatal_disable, the
timeout period should be configured to 0x0 (timeout
disabled)

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Dennis Li <Dennis.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>