drm/i915/sbi: add a dedicated mutex for LPT/WPT IOSF sideband

The LPT/WPT IOSF sideband is unrelated to pcode or VLV/CHV IOSF
sideband. It's just confusing to piggyback on the same mutex. Add a
dedicated lock with init and cleanup functions.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/139933094fe51fccdbe14d4e20df69782e801905.1730193891.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>