drm/i915: Remove vestigal i915_gem_context locals from cmdparser

The use GEM context itself was removed in commit cd30a503
("drm/i915/gem: Excise the per-batch whitelist from the context"), but
the locals were left in place as an oversight. Remove the parameters and
clean up.

References: cd30a503 ("drm/i915/gem: Excise the per-batch whitelist from the context")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204232616.94397-1-chris@chris-wilson.co.uk