drm/i915: Remove identical mmio read functions

It is now obvious VLV, CHV and Gen9 mmio read fcuntions are
completely identical so we can remove the three copies and
just keep the newly named generic implementation.

v2: Use fwtable naming consistently. (Joonas Lahtinen)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>