drm/i915: rip out dp port enabling cludges^Wchecks

These have been added because dp links are fiddle things and don't
like it when we try to re-train an enabled output (or disable a
disabled output harder). And because the crtc helper code is
ridiculously bad add tracking the modeset state.

But with the new code in place it is simply a bug to disable a disabled
encoder or to enable an enabled encoder again. Hence convert these to
WARNs (and bail out for safety), but flatten all conditionals in the
code itself.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>