drm/omap: fix missing disable for unused encoder

When an encoder is no longer connected to a crtc, the driver will leave
the encoder enabled.

This patch adds code to track the encoder used for a crtc, and when the
encoder changes, the old one is disabled.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>