drm/radeon/kms: change Kconfig text to reflect the new option.

Ingo pointed out that we really don't give the user enough warning to make
a decision here. So revise the Kconfig text with a better warning.

Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Dave Airlie <airlied@redhat.com>