cxl/pci: Delay event buffer allocation

cxl/pci: Delay event buffer allocation

The event buffer does not need to be allocated if something has failed in
setting up event irq's.

In prep for adjusting event configuration for DCD events move the buffer
allocation to the end of the event configuration.

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Li Ming <ming4.li@intel.com>
Link: https://lore.kernel.org/all/663922b475e50_d54d72945b@dwillia2-xfh.jf.intel.com.notmuch/
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20241107-dcd-type2-upstream-v7-4-56a84e66bc36@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>