drm/i915/pps: add higher level intel_pps_init() call

Add a new init call to be called only once, unlike some of the other
various init calls. This lets us hide more functions within
intel_pps.c. No functional changes.

Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/290865ed9b0ea79120222a24c233a2d596239076.1610127741.git.jani.nikula@intel.com