drm/i915: ignore bios output config if not all outputs are on

Both Ville and QA rather immediately complained that with the new
initial_config logic from Jesse not all outputs get enabled. Since the
fbdev emulation pretty much tries to always enable as many outputs as
possible (it even has hotplug handling and all that) fall back if more
outputs could have been enabled.

v2: Fix up my confusion about what enabled means - it's passed from
the fbdev helper, we need to check for a non-zero connector->encoder
link. Spotted by Ville.

v3: Add some debug output as requested by Jesse for debugging fallback
issues.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75552


Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>