drm/radeon: Turn off CRTCs on driver unload

radeon leaks a runtime pm ref if at least one CRTC is enabled on unload.
The ref is taken by radeon_crtc_set_config() and held as long as a CRTC
is in use. Fix by turning off all CRTCs on unload.

Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/f706b9db319e2bdaf153966a2b95a5d80f67e09b.1465392124.git.lukas@wunner.de