drm/i915: Remove i915.enable_ppgtt override

Now that we are confident in providing full-ppgtt where supported,
remove the ability to override the context isolation.

v2: Remove faked aliasing-ppgtt for testing as it no longer is accepted.
v3: s/USES/HAS/ to match usage and reject attempts to load the module on
old GVT-g setups that do not provide support for full-ppgtt.
v4: Insulate ABI ppGTT values from our internal enum (later plans
involve moving ppGTT depth out of the enum, thus potentially breaking
ABI unless we document the current values).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926201222.5643-1-chris@chris-wilson.co.uk