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

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

commit 87364760de5d631390c478fcbac8db1b926e0adf 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>
Signed-off-by: Andi Kleen <ak@linux.intel.com>