drm/i915: Avoid nesting of domain changes when setting display plane

Nesting domain changes will cause confusion when trying to interpret the
tracepoints describing the sequence of changes for the object, as well
as obscuring the order of operations for the reader of the code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>