drm/i915/adlp: Assert that VMAs in DPT start at 0

Atm the DPT object can accommodate only one VMA, so the VMA offset will
be always 0. Add an assert for this.

Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210906182715.3915100-4-imre.deak@intel.com