drm/i915/pps: split out PPS regs to a separate file

Clean up i915_reg.h by splitting out PPS regs to
display/intel_pps_regs.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/80d66ee6d7e56153a0ab25640ac2dad239b1ef6e.1678973282.git.jani.nikula@intel.com