drm/radeon: make cp variable an array

Replace cp, cp1 and cp2 members with just an array
of radeon_cp structs.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>