drm/radeon: set si_notify_smc_display_change properly

This is effectively a revert of 4573388c.

Forcing a display active when there is none causes problems with
dpm on some SI boards which results in improperly initialized
dpm state and boot failures on some boards.  As for the bug commit
4573388c tried to address, one can manually force the state to
high for better performance when using the card as a headless compute
node until a better fix is developed.

bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=73788
https://bugs.freedesktop.org/show_bug.cgi?id=69395



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
cc: stable@vger.kernel.org