drm/xe: Drop xe_mmio_write64()

drm/xe: Drop xe_mmio_write64()

The only possible 64-bit register writes in the driver come from the
highly questionable MMIO ioctl.  That ioctl's register write support
only operates for userspace running as root and cannot be used by any
real userspace; it exists solely to support the "xe_reg" debug tool in
IGT.  Since the spec indicates that hardware does not officially support
64-bit register accesses, there's no reason to allow such 64-bit writes,
even for debugging.

Bspec: 60027
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://lore.kernel.org/r/20230823003312.1356779-4-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>