Fix incorrect handling of Debug Marker functions

Fix incorrect handling of Debug Marker functions

The implementation for VK_KHR_maintenance_5 exposed a bug in how the function
for debug_marker functions were gotten. The bug was to only check if a
driver supports a physical device when querying for device function pointers,
which ignores the possibility of a layer supporting a device extension. This
is an issue for VK_EXT_debug_marker, which requires trampoline functions to
properly unwrap data passed in, resulting in the various debug marker functions
returning NULL instead of a valid function pointer.