drm/xe: Protect devcoredump access after unbind

While we don't have the full flow protection when devcoredump
is accessed after device unbind. Let's at least for now
protect against null dereference:

[  422.766508] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[  423.119584] RIP: 0010:xe_vm_snapshot_free+0x30/0x180 [xe]

While at it, I also fixed a non-standard code-declaration block
on the similar function of xe_guc_submit.

v2: - Use IS_ERR_OR_NULL (Nirmoy)
    - Expand to other functions

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240403195044.239766-1-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>