wgl: do not create screen from DllMain

wgl: do not create screen from DllMain

There's a lot of operations that aren't allowed from DllMain, so we
shouldn't create a driver-screen from there. So let's instead delay this
until it's needed from a normal function call.

See https://docs.microsoft.com/en-us/windows/win32/dlls/dllmain for
details about what is allowed and isn't from DllMain.

Reviewed-by: Neha Bhende <bhenden@vmware.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4307>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4307>