drm/i915/sbi: add intel_sbi_{lock,unlock}()

Abstract the LPT/WPT IOSF sideband locking by adding dedicated sbi
lock/unlock functions.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/61929c2fad4d4ff64e57ea2a28007f2efeb5113c.1730193891.git.jani.nikula@intel.com


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