drm/i915/clock: mass rename dev_priv to i915

Follow the contemporary naming style. Include some indentation fixes
while at it on the affected statements.

One function needs to keep using dev_priv due to implicit dev_priv usage
in a macro.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230403122428.3526263-2-jani.nikula@intel.com