drm/i915: Commit skl+ planes in an order that avoids ddb overlaps

skl+ can go belly up if there are overlapping ddb allocations between
planes. If we could absolutely guarantee that we can perform the atomic
update within a single frame we shouldn't have to worry about this. But
we can't rely on that so let's steal the ddb overlap check trick from
skl_update_crtcs() and apply it to the plane updates. Since each step
of the sequence is free from ddb overlaps we don't have to worry about
a vblank sneaking up on us in the middle of the sequence. The partial
state that gets latched by the hardware will be safe. And unlike
skl_update_crtcs() we don't have to intoduce any extra vblank waits
on account of only having to worry about a single pipe.

v2: Fix typo in commit msg (Matt)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114210729.16185-12-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>