drm/i915: Clean up PCI config register handling

Do not use magic numbers, do not prefix stuff with "PCI_", do not
declare registers in implementation files. Also move the PCI
registers under correct comment in i915_reg.h.

v2:
- Consistently use BSM (not BDSM or other variants from PRM) (Chris)
- Also include register address to help identify the register (Chris)
v3:
- Refer to register value as *_val instead of *_reg (Chris)
v4:
- Make style checker happy

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>