drm/i915/dsb: split out DSB regs to a separate file

Clean up i915_reg.h by splitting out DSB regs to
display/intel_dsb_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/d74b3c564b2d080bf689b3360f1a5e62e47f2e7c.1678973283.git.jani.nikula@intel.com