e_util_transform: fix flickering when client attach null buffer

e_util_transform: fix flickering when client attach null buffer

client attach null buffer when it want to unmap
size of ec is updated by size of buffer and core transform use
size of ec.
so ec is transformed by 0x0 size when window  is unmapped and
core transform is enabled.
it cause flickering frame when client attach valid buffer after
it is unmapped.

ex) window is 1920x1080 and core transform size is 3840x2160
1. client attach null buffer and it is umapped
2. transform is updated by 0x0 size because size of ec is 0x0
3. but ec is visible because hide effect
3. client is rendered with transform 0x0 size
4. client attach 1920x1080 size buffer and transform is updated
   by 3840x2160
5. client is rendered with transform 3840x2160 size

Change-Id: Icd74973ae3bd18fda47fcadc47d9fad37bc667a5