egl: fix _eglQuerySurface in EGL_BUFFER_AGE_EXT case

egl: fix _eglQuerySurface in EGL_BUFFER_AGE_EXT case

Specification states that in case of error, value should not be
written, patch changes buffer age queries to return -1 in case of
error so that we can skip changing the value.

In addition, small change to droid_query_buffer_age to return 0
in case buffer does not have a back buffer available.

Fixes:
   dEQP-EGL.functional.negative_partial_update.not_postable_surface

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>
Cc: mesa-stable@lists.freedesktop.org