surfaceflinger: add RenderEngine::mEGLDisplay

It does not make much sense to have EGLConfig/EGLContext but not
EGLDisplay in RenderEngine.  We also need this as we move forward on
making EGL an implementation detail of RenderEngine.

Test: SurfaceFlinger_test
Change-Id: I9e2c9b9817e2c2a9b2555ea575be6dcd91927b79
3 files changed