drm/i915/aux: split out DP AUX regs to a separate file

Clean up i915_reg.h by splitting out DP AUX regs to
display/intel_dp_aux_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/aa93b34e786c5566acf8f053ffed96c160a23898.1678973282.git.jani.nikula@intel.com