drm/amdgpu: enable mmhub power gating

Remove some functions due to the design change.
All the mmhub power gating sequence is moved to
smu fw.Driver sends the message to enable mmhub
powergating.We can also skip the fw version check
since the old fw version is in a very early stage
and we don't use that fw for release.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>