drm/radeon: add locking around atombios scratch space usage

While developing MST support I noticed I often got the wrong data
back from a transaction, in a racy fashion. I noticed the scratch
space wasn't locked against concurrent users.

Based on a patch by Alex, but I've made it a bit more obvious when
things are locked.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

diff --cc drivers/gpu/drm/radeon/atom.c
Simple merge

diff --cc drivers/gpu/drm/radeon/atom.h
Simple merge

diff --cc drivers/gpu/drm/radeon/atombios_dp.c
Simple merge

diff --cc drivers/gpu/drm/radeon/atombios_i2c.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_device.c
Simple merge