drm/i915: consolidate and tighten encoder cloning checks

drm/i915: consolidate and tighten encoder cloning checks

Only lvds/tv did actually check for cloning or not, but many more
places should.

Notices because my ivb tried to enable both cpu edp and vga on the
first crtc - the resulting confusion between has_pch_encoder,
has_dp_encoder but not actually being a pch dp encoder resulting in
hilarity (hitting a BUG).

We _really_ need an igt to random-walk our modeset space more
exhaustively.

The bug seems to have been exposed due to a race in the hw load
detection support for VGA: Right after a hotplug VGA was still
detected as connected, but obviously reading the EDID wasn't possible
any more. Hence why restarting X a bit later fixed things. Due to the
1024x756 fallback resolution suddenly more outputs had the same
resolution.

On top of that SNA was confused with the possible_clones mask, trying
to clone outputs which cannot be cloned. That bug is now fixed with

commit fc1e0702b25e647cb423851fb7228989fec28bd6
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed May 29 11:25:28 2013 +0100

    sna: fixup up possible_clones kms->X impedance mismatch

v2: Kill intel_encoder_check_is_cloned, spotted by Paulo.

v3: Drop the now unused pipe param.

v4: Kill the stray printk Chris spotted.

v5: Elaborate on how the bug in userspace happened and why it was racy
to reproduce.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>