drm/radeon/kms: add crtc_disable function for legacy crtc

To plug the VRAM memory leak (see previous patch for
details) we must unpin the frame buffer when disabling the
CRTC. This warrants the addition of disable function for legacy
CRTC, which puts the CRTC in DPMS-OFF state and unpins the
frame buffer if there is one associated with the CRTC.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>