vk/runtime: Allow enumerate and try_create_for_drm to coexist

For drivers that can support both drm and non-drm kernel mode drivers it
is useful to be able to provide both entrypoints.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21394>