drm/i915/pps: move pps code over from intel_display.c and refactor

intel_display.c has some pps functions that belong to intel_pps.c. Move
them over.

While at it, refactor the duplicate intel_pps_init() in intel_display.c
into an orthogonal intel_pps_setup() in intel_pps.c, and call it earlier
in intel_modeset_init_nogem().

Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210120101834.19813-2-jani.nikula@intel.com