gralloc: Override gralloc calculated size only if bufferSize is greater.

bufferSize set using native window set buffer size will override gralloc
calculated size only if bufferSize is greater.
Fix for 720P, 1080P HD content playback is not working on 7x27A due to
buffer size mismatch between requested size by frameworks and allocated
size by gralloc for HAL_PIXEL_FORMAT_YCrCb_420_SP and
HAL_PIXEL_FORMAT_YCrCb_420_SP format

Change-Id: I16d04e406ab8f1f35e0e0a70e19d99a019adc5ef
CRs-Fixed: 451783