drm/xe/vf: Don't change hwe IRQ masks if using memory IRQs

We currently do not support changing the engine interrupt enable
mask on the per-engine basis when using memory based interrupts.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619214557.905-5-michal.wajdeczko@intel.com