drm/amdgpu: change default behavior of bad_page_threshold parameter

Ignore ras umc bad page threshold by default, GPU initialization won't
be stopped in this mode.

v2: refine the description of bad_page_threshold.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Stanley.Yang <Stanley.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>