drm/xe/mmio: fix forcewake ref leak in xe_mmio_ioctl

Make sure we properly release the forcewake ref on all error paths.

v2(Lucas):
  - Make it less verbose and just fold the unimplemented options into
    the default. The exact return value doesn't seem to matter for the
    corresponding IGT.
  - Replace the user triggerable WARN() with drm_dbg().

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>