drm/i915: fix kernel-doc warnings in i915_gem.c

Fix the following 'make htmldocs' warnings:

  .//drivers/gpu/drm/i915/i915_gem.c:1729: warning: No description found for parameter 'vma'
  .//drivers/gpu/drm/i915/i915_gem.c:1729: warning: No description found for parameter 'vmf'

  .//drivers/gpu/drm/i915/i915_gem.c:4962: warning: No description found for parameter 'old'
  .//drivers/gpu/drm/i915/i915_gem.c:4962: warning: No description found for parameter 'new'
  .//drivers/gpu/drm/i915/i915_gem.c:4962: warning: No description found for parameter 'frontbuffer_bits'

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>