drm/i915: stop checking for NULL vma->obj

This is no longer possible since e6e1a304 ("drm/i915: vma is always
backed by an object.").

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304174252.1000238-1-matthew.auld@intel.com