drm/radeon/kms/pm: rework power management

Add two new sysfs attributes:
- dynpm
- power_state

Echoing 0/1 to dynpm disables/enables dynamic power management.
The driver scales the sclk dynamically based on the number of
queued fences.  dynpm only scales sclk dynamically in single head
mode.

Echoing x.y to power_state selects a static power state (x) and clock
mode (y).  This allows you to statically select a power state and clock
mode.  Selecting a static clock mode will disable dynpm.

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