drm/radeon/kms: simplify storing current and requested PM mode

We kept pointers to requested and current clock modes in every power state.
That was useless, more /global/ pointers in power struct are enough.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>