drm/xe: correct the assertion for number of PTEs

While one MI_STORE_DATA_IMM can take no more than 0x1fe qwords,
the size of the pgtable can be 512 entries.

Fixes: 43d48379 ("drm/xe: correct the calculation of remaining size")
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Fei Yang <fei.yang@intel.com>
Tested-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240125065245.1204731-2-fei.yang@intel.com