drm/radeon/dpm: add pre/post_set_power_state callbacks (6xx-eg)

For r6xx-evergreen, they are no-ops as they don't support
any dynamic state adjustment.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/r600_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.h
Simple merge