drm/i915: split out vlv sideband registers from i915_reg.h

drm/i915: split out vlv sideband registers from i915_reg.h

Add a dedicated file vlv_sideband_reg.h for the VLV/CHV sideband
registers. The sideband registers macros are needed by the same files
that need vlv_sideband.h, so include the definitions from there.

v2: Fix copyright year (Matt)

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220110095740.166078-2-jani.nikula@intel.com