drm/radeon/kms: add dynamic engine reclocking (V9)

V2: reorganize functions, fix modesetting calls
V3: rebase patch, use radeon's workqueue
V4: enable on tested chipsets only, request VBLANK IRQs
V5: enable PM on older hardware (IRQs, mode_fixup, dpms)
V6: use separate dynpm module parameter
V7: drop RADEON_ prefix, set minimum mode for dpms off
V8: update legacy encoder call, fix order in rs600 IRQ
V9: update compute_clocks call in legacy, not only DPMS_OFF

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