drm/i915: Idle unused rings on gen2/3 during init/resume

gen2/3 platforms have a boatload of rings we're not using. On my 830
the BIOS/hw can leave some of those "active" after resume which will
prevent c3 entry. The ring is apparently considered active whenever
head != tail even if the ring is disabled.

Disable and clear all such unused ringbuffers on init/resume.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/i915_gem.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_reg.h
Simple merge