drm/radeon/dpm: save some display parameters for DPM

Required for SI.

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

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

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

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