egl: fold _eglError() + return EGL_FALSE

egl: fold _eglError() + return EGL_FALSE

The function _eglError() already explicitly returns EGL_FALSE,
explicitly to simplify the callers. Make use of it.

While EGL_FALSE is numerically identical to false, NULL, EGL_NO_FOO,
storage is not the same so we cannot use it for "everything".

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>