i915: revert some checks added by commit 32aad86f

This fixes blur-like screen corruption on the following card:

VGA compatible controller [0300]: Intel Corporation 82G33/G31 Express
Integrated Graphics Controller [8086:29c2] (rev 10)

intel_sdvo_mode_set() should not return prematurely just because some
features are not supported.

https://bugzilla.kernel.org/show_bug.cgi?id=17151

Signed-off-by: Pavel Roskin <proski@gnu.org>
Reported-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[ickle: Relax a couple more checks for failing LVDS modesetting]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>