drm/i915: Hook up ring workaround writes at context creation time on Gen6-7.

intel_rcs_ctx_init() emits all workaround register writes on the list
to the ring, in addition to calling i915_gem_render_state_init().  The
workaround list is currently empty on Gen6-7 so this shouldn't cause
any functional changes.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>