drm/i915: fix startup hang on some non-mobile platforms

Due to a bogus FBC support check and failing to check for FBC support
in the right places, mode setting on non-mobile platforms could fail
and hang in the FBC disable routine.  Fix it up.

This fix highlights the need for cleanups in this area (function
pointers and better feature support checks).  Patches for that to
follow.

Tested-by: Kenny Graunke <kenny@whitecape.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>