[OpenCL] Undefine cl_intel_planar_yuv extension

Summary:

Remove unnecessary definition (otherwise the extension will be defined
where it's not supposed to be defined).

Consider the code:

  #pragma OPENCL EXTENSION cl_intel_planar_yuv : begin
  // some declarations
  #pragma OPENCL EXTENSION cl_intel_planar_yuv : end

is enough for extension to become known for clang.

Patch by: Dmitry Sidorov <dmitry.sidorov@intel.com>

Reviewers: Anastasia, yaxunl

Reviewed By: Anastasia

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58666

llvm-svn: 362398