drm/i915/lrc: Update PDPx registers with lri commands

A safer way to update the PDPx registers is sending lri commands, added
in the ring before the batchbuffer start. Otherwise, the ctx must be idle
before trying to change anything (but the ring-tail) in the ctx image. An
example where the ctx won't be idle is lite-restore.

This patch depends on 5b7e4c9c ("drm/i915/gtt: Mark TLBS dirty for gen8+").

v2: Combine lri writes (and save 8 commands). (Mika)
v3: Rebase after ring/req changes, and removed references to deprecated patches.

Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>