drm/i915/crt: drop a bunch of unnecessary register variables

There's no need to save the register offsets. Drop the variables.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3493286ecd1ae166e1e15235d31115f766f7c878.1670433372.git.jani.nikula@intel.com