Invoke a padding round in certain use-cases.

* In some use-cases, it is possible that there are
no AppBuffer layers on the external/virtual layer-list
during which all the pipes will be allocated to the
primary. When layers do comeup on external/virtual
layer-list, subsequent overlay sets fail.

* This change ensures that in such cases, we invoke a
padding round on all the displays to free up the
hw resources which can be used in subsequent cycles.

Change-Id: Ifac0b8f51a8719eb55b11010d05b8d11352db054
gralloc: Set default value for implementation defined formats

For implementation defined formats which go via HWC, set the
default value to RGBA8888

Change-Id: Ia1abcbd74c8b44baee34f39cda28740031a64b7c