drm/i915: Init some CHV workarounds via LRIs in ring->init_context()

Follow the BDW example and apply the workarounds touching registers
which are saved in the context image through LRIs in the new
ring->init_context() hook.

This makes Mesa much happier and eg. glxgears doesn't hang after
the first frame.

Cc: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Add missing wa table initialization to avoid a functional
conflict with Arun's wa table debugfs support.]
Reviewed-by: "Barbalho, Rafael" <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>