drm_hwcomposer: Remove writeback-based flattening logic

Writeback Pros:
a. Very tiny reduction in power consumption required for
   rendering the composition compared to CLIENT-based flattening.

Writeback Cons:
a. A lot of extra code. High maintenance cost(effort).
   Nobody tests it as for now.
b. Writeback isn't implemented by the set of KMS drivers
c. Require separate memory buffer to be allocated

Part of: drm-hwcomposer/drm-hwcomposer#47
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
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