drm/xe: Fix checking for unset value

drm/xe: Fix checking for unset value

Commit 37430402618d ("drm/xe: NULL binding implementation") introduced
the NULL binding implementation, but left a case in which the out value
is_vram is not set and the caller will use whatever was on stack.
Eventually the is_vram out could be removed, but this should at least
fix the current bug.

Fixes: 37430402618d ("drm/xe: NULL binding implementation")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230726160708.3967790-4-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>