[Intel-gfx] [PATCH] drm/atomic-helper: Fix kerneldoc for prepare_planes

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 5838
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  282/282              282/282
ILK                                  308/308              308/308
SNB                                  326/326              326/326
IVB                                  379/379              379/379
BYT                                  294/294              294/294
HSW                                  387/387              387/387
BDW                 -1              316/316              315/316
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*BDW  igt_gem_gtt_hog      PASS(13)      DMESG_WARN(1)PASS(1)
Note: You need to pay more attention to line start with '*'