drm/crtc-helpers: Enforce sane set_config api

There's no point in trying to clean up after driver-bugs, so just blow
up. Furthermore it's an interface abuse to set no mode but have an fb
and aslo to try to set an fb without enough connectors. These two
spefici cases of interface abuse have been committed by the fb helper,
but that's been fixed meanwhile in

commit 7e53f3a4
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Jan 21 10:52:17 2013 +0100

    drm/fb-helper: fixup set_config semantics

The i915 driver has been shipping since a while with these BUGs with
no reports, so should be save.

Note that this drops an ugly case where we clear crtc->fb behind the
upper levels back and so cause a refcounting mayhem, which Russell
Kins spotted while trying to hunt down a drm framebuffer leak.

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>