drm/amdgpu: Turn off CRTCs on driver unload

amdgpu leaks a runtime pm ref if at least one CRTC is enabled on unload.
The ref is taken by amdgpu_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/7bf8d9ceb9d343a7495788667e6da170b8fd3af1.1465392124.git.lukas@wunner.de