drm/i915: Don't oops if the initial modeset fails

If the first modeset operation fails, we will attempt to restore the
previous configuration that we read out from the hardware. But as we
don't yet reconstruct the framebuffer information, we end up calling
the modeset code with an enabled crtc but with fb==NULL. This will
lead to an oops within the modeset code.

Check for NULL fb when restoring the configuration, and instead of
oopsing simply disable the pipe.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>