drm: fix drm_fb_helper handling of kernel crtcs

The drm_fb_helper shouldn't mess with CRTCs that aren't enabled or in
its initial config. Ideally it shouldn't even include CRTCs in its
initial config if they're not in use, but my old fix for that no longer
works.  At any rate, this fixes a real bug I was seeing where after a
console blank, both pipes would come back on, even though only one had
been enabled before that.  Since the other pipe had a bogus config,
this led to some screen corruption.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>