hwc: Use GPU to compose for smaller updates

Whether or not an update is small (conservatively set to < 5% of
screen) is checked using total dirty areas of layers and GPU is
used to compose. This is because a smaller update also means GPU
had less front end work and the MDP can avoid fetching data from
all layers until idle timeout.

Change-Id: I1ccd8913f2a1c760f570b225fdfb29f6a20b5a8d
1 file changed