drm/xe: Introduce const cast helper

Typically we want to preserve pointer constness when converting
from one xe pointer to another, but in some rare cases, like kunit
parameter conversions, we might want to discard this constness.
Add a helper that we will use to clearly indicate our intention.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com> #v1
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240720142528.530-2-michal.wajdeczko@intel.com