drm/i915/gen9: Re-allocate DDB only for changed pipes

When a display update triggers a DDB re-allocation, we should start by
assuming that only the updated pipes need to be re-allocated (we have
logic later that may add additional pipes if, e.g., a modeset triggers a
change to the global allocation).

We were erroneously using the _active_ pipes as our starting point
rather than the changed pipes.  This causes us to grab CRTC locks that
we didn't actually need, reducing parallelism.  Given the recent
non-blocking atomic changes, it also causes legacy pageflips against one
CRTC to return -EBUSY if there's an outstanding pageflip against a
different CRTC (a situation easily triggered via compositors like
Weston).

Fixes: 98d39494d3 ("drm/i915/gen9: Compute DDB allocation at atomic check time (v4)")
Cc: Maarten Lankhorst <[email protected]>
Signed-off-by: Matt Roper <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Maarten Lankhorst <[email protected]>

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