drm/i915: Use of BARs names instead of numbers

At the moment, when we refer to some PCI BAR we use the number of
this BAR in the code. The meaning of BARs between different platforms
may be different. Therefore, in order to organize the code,
let's start using defined names instead of numbers.

v2: Add lost header in cfg_space.c

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220805155959.1983584-2-piotr.piorkowski@intel.com