drm/amdgpu: some improvement in parsing inputs

It changes the way to skip newline character and also avoids
warning message from some compiler.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>