drm/i915: Fix kerneldoc warnings in i915_gem_execbuffer

drivers/gpu/drm/i915/i915_gem_execbuffer.c:1983: warning: No description found for parameter 'dev_priv'
drivers/gpu/drm/i915/i915_gem_execbuffer.c:1983: warning: No description found for parameter 'file'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208113917.8439-1-chris@chris-wilson.co.uk