SurfaceFlinger: use HAS_CONTEXT_PRIORITY

The OMAP3 SGX530 EGL driver has this extension, and it appears to help
performance, so use it.

Change-Id: I5019674a3199e1efa69ccf57c8f860abdab645f9
libgui: don't use EGL_KHR_fence_sync

The EGL implementation for OMAP3/SGX530 has this extension, but
testing shows that it causes stuttering in the UI.

Change-Id: If750df1bc4730e5c27767930af9da89630f0ff1e