drm/i915: compute masks of crtcs affected in set_mode

This is definetely a bit more generic than currently required, but if
we keep track of all crtcs that need to be disabled/enable (because
they loose an encoder or something similar), crtcs that get completely
disabled and those that we need to do an actual mode change nicely
prepares us for global modeset operations on multiple crtcs.

The only big thing missing here would be a global resource allocation
step (for e.g. pch plls), which would equally frob these bitmasks if
e.g. a crtc only needs a new pll. Or if we need to enable dithering on
an another pipe due to bandwidth constrains somewhere.

These masks aren't yet put to use in this patch, this will follow in the
next one.

v2-v5: Fix up the computations for good (hopefully).

v6: Fixup a confusion reported by Damien Lespiau: I've conserved the
(imo braindead) behaviour of the crtc helper to disable _any_
disconnected outputs if we do a modeset, even when that newly disabled
connector isn't connected to the crtc being changed by the modeset.

The effect of that is that we could disable an arbitrary number of
unrelated crtcs, which I haven't taken into account when writing this
code. Fix this up.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>