Output::presentFrameAndReleaseLayers: flush pending commands for OFF displays

Most work from this method can be skipped if the display is not enabled.
However, uncaching buffers should still occur. If the display is
disabled and there are buffers to uncache, still flush pending commands.
They should only contain commands that are meaningful for a disabled
display, like bufferSlotsToClear.

Bug: 330806421
Test: libcompositionengine_test
Flag: flush_buffer_slots_to_uncache
Change-Id: I7baa3e76af86329fb266395e63e92a0ba38967f4