wl_egl: improve end-of-use guarantee for previous buffers

wl_egl: improve end-of-use guarantee for previous buffers

 AS-IS
 - Dequeue thread waits for the wl-egl-thread to send signal
 after commit the buffer that was last enqueued.
 PROBLEMS
 - Complex mutex locking is required to access last_enq_buffer.
 - Complex process is required to validate last_enq_buffer.
 TO-BE
 - The list of buffers in use is traversed by wl-egl-thread
 and signal is sent when all buffers enqeueued after queue reset
 are committed.

Change-Id: Ib81186c2cc1faf856372c0a668b47ffc615258eb
Signed-off-by: Joonbum Ko <joonbum.ko@samsung.com>