drm/amd/pm/si: Delete unused structs and fields

drm/amd/pm/si: Delete unused structs and fields

The contents of si_dpm.h seem to have been copied from the
old radeon driver, including a lot of structs and fields which
were only relevant to GPU generations even older than SI.

A lot of these can be deleted without causing much churn to the
actual SI DPM code. Let's delete them to make the code easier
to understand.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>