drm/ast: Don't check new mode if CRTC is being disabled

drm/ast: Don't check new mode if CRTC is being disabled

Suspending failed because there's no mode if the CRTC is being
disabled. Early-out in this case. This fixes runtime PM for ast.

v3:
	* fixed commit message
v2:
	* added Tested-by/Reported-by tags
	* added Fixes tags and CC (Sam)
	* improved comment

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Cary Garrett <cogarre@gmail.com>
Tested-by: Cary Garrett <cogarre@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 8a7e5eddfb5b ("drm/ast: Add CRTC helpers for atomic modesetting")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: <stable@vger.kernel.org> # v5.6+
Link: https://patchwork.freedesktop.org/patch/msgid/20200507090640.21561-1-tzimmermann@suse.de