drm/i915/pps: refactor init abstractions

Once you realize there is no need to hold the pps mutex when calling
pps_init_timestamps() in intel_pps_init(), we can reuse
intel_pps_encoder_reset() which has the same code.

Since intel_dp_pps_init() is only called from one place now, move it
inline to remove one "init" function altogether.

Finally, remove some initialization from
vlv_initial_power_sequencer_setup() and do it in the caller to highlight
the similarity, not the difference, in the platforms.

v2: Fix comment (Anshuman)

Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210120101834.19813-1-jani.nikula@intel.com