drm/i915: Remove leftover vma->obj->pages_pin_count on insert/remove

We now do the page pin count upfront in vma_get_pages/vma_put_pages, so
that we do the allocations before we enter the vm->mutex. Our vma
page references we are tracked in vma->pages_count and the extra
obj->pages_pin_count being performed later in i915_vma_insert and
i915_vma_remove is redundant, and worse throws off the shrinker's logic
on when it can free an object by unbinding it.

Reported-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reported-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015100155.10376-1-chris@chris-wilson.co.uk