drm/i915/gem: Only call eb_lookup_vma once during execbuf ioctl

As we no longer stash anything inside i915_vma under the exclusive
protection of struct_mutex, we do not need to revoke the i915_vma
stashes before dropping struct_mutex to handle pagefaults. Knowing that
we must drop the struct_mutex while keeping the eb->vma around, means
that we are required to hold onto to the object reference until we have
marked the vma as active.

Fixes: 155ab883 ("drm/i915: Move object close under its own lock")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303204345.1859734-3-chris@chris-wilson.co.uk