drm_hwcomposer: Fix BufferInfoMaliMeson

In commit e9fbd8d6 ("drm_hwcomposer: Set return type to
std::optional for BufferInfoGetters") the final return for
BufferInfoMaliMeson::GetBoInfo() erroneously was changed to
return {} instead of the calculated BufferInfo object.

This simply fixes the issue, returning the BufferInfo.

Change-Id: I03789b48dc4a0df64432f7ff31661746163a96b8
Signed-off-by: John Stultz <jstultz@google.com>
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