drm/xe/pf: Fix migration initialization

The migration support only needs to be initialized once, but it
was incorrectly called from the xe_gt_sriov_pf_init_hw(), which
is part of the reset flow and may be called multiple times.

Fixes: d86e3737 ("drm/xe/pf: Add functions to save and restore VF GuC state")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250120232443.544-1-michal.wajdeczko@intel.com