drm/xe: Use xe_fb_pin_ggtt_pin() for the initial FB pin

drm/xe: Use xe_fb_pin_ggtt_pin() for the initial FB pin

Use xe_fb_pin_ggtt_pin() instead of intel_fb_pin_to_ggtt() for
the initial FB pin. We want to get rid of intel_fb_pin_to_ggtt()
and just use the new fb_pin parent interface.

This still isn't quite the final solution since we bypass the
actual parent interface and call the implementation directly.
But sorting that out will require more cleanup to the initial
FB code.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260508143426.26504-16-ville.syrjala@linux.intel.com