drm/i915: Remove PIN_UPDATE for i915_vma_pin

As we no longer use PIN_UPDATE (since commit 7d0aa0db

 ("drm/i915/gem:
Unbind all current vma on changing cache-level")) we can remove
PIN_UPDATE itself. The benefit is just in simplifing the vma bind.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200521144949.25357-1-chris@chris-wilson.co.uk