drm/radeon/kms: Fix R600 write back buffer

This split write back buffer handling into 3 functions,
wb_fini for cleanup, wb_enable/wb_disable for enabling/disabling
write back used for suspend/resume. This should fix
potential issue of letting the write back active before
suspending. We need to allocate memory in wb_enable because
we can only allocate once GART is running.

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