drm/gpuvm: Do not prepare NULL objects

drm/gpuvm: Do not prepare NULL objects

Statis analysis issue:

drm_gpuvm_prepare_range issues an exec_object_prepare call to all
drm_gem_objects mapped between addr and addr + range.  However, it is
possible (albeit very unlikely) that the objects found through
drm_gpuvm_for_each_va_range (as connected to va->gem) are NULL, as seen
in other functions such as drm_gpuva_link and drm_gpuva_unlink_defer.

Do not prepare NULL objects.

Fixes: 50c1a36f594b ("drm/gpuvm: track/lock/validate external/evicted objects")
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Link: https://patch.msgid.link/20260130191953.61718-2-jonathan.cavitt@intel.com