drm/i915: Drop code to handle set-vm races from execbuf

Changing the vm from a finalized gem ctx is no longer possible, which
means we don't have to check for that anymore.

I was pondering whether to keep the check as a WARN_ON, but things go
boom real bad real fast if the vm of a vma is wrong. Plus we'd need to
also get the ggtt vm for !full-ppgtt platforms. Ditching it all seemed
like a better idea.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
References: ccbc1b97 ("drm/i915/gem: Don't allow changing the VM on running contexts (v4)")
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210902142057.929669-4-daniel.vetter@ffwll.ch