drm/i915: Remove lost comment from i915_gem_context

The comment is very old and quite misleading now.

drivers/gpu/drm/i915/i915_gem_context.c:349: warning: No description found for parameter 'dev_priv'
drivers/gpu/drm/i915/i915_gem_context.c:349: warning: No description found for parameter 'file_priv'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208111559.32663-1-chris@chris-wilson.co.uk