drm/i915: Drop unused AUX register offsets

We reference DP AUX registers via the DP_AUX_CH_CTL() and
DP_AUX_CH_DATA() macros that calculate all the register offsets for us
automatically; there's no need to explicitly define every offset in
i915_reg.h if they're never going to be used by the driver code.

v2: Apparently GVT was directly using these raw definitions in a couple
    places.  Switch GVT code over to using our preferred macros.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>  #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20191026051226.30807-1-matthew.d.roper@intel.com