drm/xe/guc: Fix inverted logic on snapshot->copy check

Currently the check to see if snapshot->copy has been allocated is
inverted and ends up dereferencing snapshot->copy when free'ing
objects in the array when it is null or not free'ing the objects
when snapshot->copy is allocated. Fix this by using the correct
non-null pointer check logic.

Fixes: d8ce1a97 ("drm/xe/guc: Use a two stage dump for GuC logs and add more info")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241009160510.372195-1-colin.i.king@gmail.com