drm/i915/gen9+: Preserve old allocation from crtc_state.

drm/i915/gen9+: Preserve old allocation from crtc_state.

This is the last bit required for making nonblocking modesets work
correctly. The state in intel_crtc->hw_ddb is updated in the
nonblocking part of a nonblocking commit.

This means that even attempting a commit before a nonblocking modeset
completes will fail, because intel_crtc->hw_ddb still has stale values.
The stale values are 0 if the crtc is being enabled resulting in a
failure during atomic check, but it may also result in double use of
ddb allocations.

Fix this by explicitly copying the ddb allocation from the old state.
This has to be done explicitly, because a modeset that doesn't change
active pipes, or a modeset converted to a fastset will will clear the
current state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478609742-13603-4-git-send-email-maarten.lankhorst@linux.intel.com
[mlankhorst: Reword commit message.]
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>