drm/radeon/kms: fix s/r issues with bios scratch regs

commit 87364760 upstream.

The accelerate mode bit gets checked by certain atom
command tables to set up some register state.  It needs
to be clear when setting modes and set when not.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=26942



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>