drm/radeon/kms: Workaround RV410/R420 CP errata (V3)

Long story short, this fixes sporadic hardlocks with my rv410 during
times of intense 2D acceleration (Flash on Fx3).

V2: Fix indentation and move errata_fini to suspend function so we
don't leak scratch register over suspend/resume cycle.
V3: Move scratch_reg to asic specific structure (aim is to slowly
    move stuff to asic specific structure and avoid poluting
    radeon_device struct with asic specific variables)

Signed-off-by: Corbin Simpson <MostAwesomeDude@gmail.com>
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>