drm/i915: vma is always backed by an object.

vma->obj and vma->resv are now never NULL, and some checks can be removed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211117142024.1043017-4-matthew.auld@intel.com