egl/android: Stop leaking DRI images

Current implementation of the DRI image loader does not free the images
created in get_back_bo() and so leaks memory. Moreover, it creates a new
image every time the DRI driver queries for buffers, even if the backing
native buffer has not changed. leaking memory again.

This patch adds missing call to destroyImage() in droid_enqueue_buffer()
and a check if image is already created to get_back_bo() to fix the
above.

Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>