drm/radeon/kms: Don't overwrite crtc_gen_cntl or crtc_gen_cntl2

Don't overwritte  crtc_gen_cntl or crtc_gen_cntl2 or we may loose the
cursor. This especialy happen when changing video mode. Fix bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=529146



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