drm/amdgpu: implement read_sensor() for pre-powerplay chips

Add the GPU temperature, the shader clock and eventually the
memory clock (as well as the GPU load on CI). The main goal is
to expose this info to the userspace like Radeon.

v2: - add AMDGPU_PP_SENSOR_GPU_LOAD on CI
    - update the commit description

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>