drm/xe: Define xe_reg_is_valid

Add a function to check if struct xe_reg has valid address.

v2:
 - Rebase.
 - Make xe_reg_is_valid as inline function instead of a macro. (Badal).
 - Update commit msg.

Signed-off-by: Karthik Poosa <karthik.poosa@intel.com>
Suggested-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Badal Nilawar <badal.nilawar@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240405130127.1392426-2-karthik.poosa@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>