drm/i915/ipc: refactor and rename IPC functions

Rename the IPC functions to have skl_watermark_ipc_ prefix, rename
enable to update to reflect what the function actually does, and add
enabled function to abstract direct ->ipc_enabled access for state
query.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/536237d5bc919e8c97a96796f235f5bb264ceff2.1662983005.git.jani.nikula@intel.com