drm/i915: use mock device info for creating mock device

Instead of modifying the device info on the fly, use static const mock
device info.

It's not okay to modify device info at runtime; we've added separate
runtime info for info that needs to be modified at runtime. We've added
safeguards to device info to prevent it from being modified, but commit
5e352e32 ("drm/i915: preparation for using PAT index") just cast the
const away and modified it anyway. This prevents device info from being
moved to rodata.

Fixes: 5e352e32 ("drm/i915: preparation for using PAT index")
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Fei Yang <fei.yang@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b0db62045a96a3fd4cf123685da88cc777f9b485.1687878757.git.jani.nikula@intel.com