drm/amd/powerplay: add vega20 pre_display_config_changed callback

drm/amd/powerplay: add vega20 pre_display_config_changed callback

fix possible handshake hang and video playback crash

Corner cases:
 - Handshake between SMU and DCE causes hangs when CRTC is not
   enabled
 - System crash occurs when starting 4K playback with Movies and TV
   in an SLS configuration

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>