drm/i915: split out PCI config space registers from i915_reg.h

The PCI config space registers don't really belong next to the MMIO
register definitions.

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/20220110095740.166078-1-jani.nikula@intel.com