drm/i915: Use true PPGTT in Gen8+ when execlists are enabled

In Gen8+, full ppgtt needs execlist, otherwise the ctx switch can hang.

Also remove the current restriction, a user should be able to explicitly set
ppgtt=2.

Note, this patch considers that execlist support has been enabled by
default on Gen8.

v2: Remove non-default restriction and clarify commit message (Daniel)

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
[danvet: s/comment/commit message/ in the commit message since that's
what Michel meant as per our irc discussion.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>