drm/kms: add crtc disable function

More explicit than dpms. Same as the encoder disable function.

Need this to explicity disconnect plls from crtcs for reuse when you
plls:crtcs ratio isn't 1:1.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>