OCL: Optimize OpenCL version of sepFilter2D

Optimize kernel, which used in most cases when used sepFilter2D function.
Instead reread some pixels from src, copy them from local buffer, which already filled. 

check_regression=*OCL*Sobel*:*OCL*Scharr*:*OCL*GaussianBlur*
test_filter=*OCL*Sobel*:*OCL*Scharr*:*OCL*GaussianBlur*
test_modules=imgproc
build_examples=OFF

http://ocl.itseez.com/intel/export/perf/pr/2897/report/