drm/xe/stolen: ignore first page for FBC

We have observed underruns on some platforms if the CFB offset is within
the first page of stolen. Just like i915 skip the first page.

v2 (Maarten)
  - Also align the start.

BSpec: 50214
Reported-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240412150301.273344-4-matthew.auld@intel.com