[Intel-gfx] [PATCH] drm/i915: Ignore pipe B active state when enabling pipe C

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 5915
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -1              282/282              281/282
ILK                                  308/308              308/308
SNB                                  307/307              307/307
IVB                 -2              375/375              373/375
BYT                                  294/294              294/294
HSW                                  385/385              385/385
BDW                                  315/315              315/315
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*PNV  igt_gem_userptr_blits_minor-unsync-normal      PASS(2)      DMESG_WARN(1)PASS(1)
*IVB  igt_drv_debugfs_reader      PASS(2)      DMESG_WARN(2)
*IVB  igt_drv_hangman_error-state-sysfs-entry      PASS(2)      TIMEOUT(2)
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