drm/exynos: Disable unused crtc planes from crtc

This patch moves the code which disables unused crtc planes from the
encoder to the crtc. Since there is a 1:1 encoder/crtc mapping in
exynos, the only valid crtc change the pre-existing code could catch is
disconnecting an active crtc from the encoder. Thus it is functionally
equivalent to just disable all planes attached to a crtc when the crtc
is disabled.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>