drm/i915: add a dedicated mutex for VLV/CHV IOSF sideband

The VLV/CHV IOSF sideband is unrelated to pcode. 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/31ccbf33c6b6114d0bcb40a2e174f19162d4e177.1730193891.git.jani.nikula@intel.com


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