drm/i915: Function per early graphics quirk

Move graphics stolen memory related early quirk into a function to
allow easy adding of other graphics quirks to fix memory maps on
machines running old BIOS versions.

While at it;
- _funcs -> _ops to follow de facto naming
- make the iteration code tad more readable
- remove unused variables

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>