drm/amdgpu: expose amdgpu_sensors on pre-powerplay chips

read_sensor() has been recently implemented for dpm based boards
which means amdgpu_sensors can now be exposed.

v2: - make sure read_sensor is not NULL on dpm chips
    - keep sanity check for powerplay chips
v3: - make sure amdgpu_dpm != 0

Cc: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>