drm/xe: Fix NULL ptr dereference in devcoredump

Kernel VM do not have an Xe file. Include a check for Xe file in the VM
before trying to get pid from VM's Xe file when taking a devcoredump.

Fixes: b10d0c5e ("drm/xe: Add process name to devcoredump")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240530203341.1795181-1-matthew.brost@intel.com