egl: Make native display detection work more than once

eglGetDisplay is awful because you have to inspect the pointer you're
given and guess what type of native display it corresponds to. We make
it worse by caching the type of the first such display we detect, so if
the second call to eglGetDisplay is to a different display type, kaboom.

Fortunately this is a problem that can be solved with the delete key.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/156