drm/xe: Improve rebar log messages

drm/xe: Improve rebar log messages

Some minor improvements to the log messages in the rebar logic:
use xe-oriented printk, switch unit from M to MiB in a few places for
consistency and use ilog2(SZ_1M) for clarity.

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patch.msgid.link/20251219211650.1908961-6-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>