libagl: eglSwapInterval fix

SW egl returned always EGL_FALSE even when the display was valid.
This made calls to eglSwapInterval to fail on HW since the wrapper
fails if either SW or HW egl fails.

Letting SW eglSwapInterval to return true when display is valid is
ok since spec says that interval parameter is silently clamped.

Change-Id: Ib4aa1dbfccf510a0f36621796e79a4d46852ee6a