drm/radeon/dce8: workaround for atom BlankCrtc table

drm/radeon/dce8: workaround for atom BlankCrtc table

commit 78fe9e545ce6d510b979dc2d8e14096a279fc519 upstream.

Some DCE8 boards have a funky BlankCrtc table that results
in a timeout when trying to blank the display.  The
timeout is harmless (all operations needed from the table
are complete), but wastes time and is confusing to users so
work around it.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=73420

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>