OCL: optimization of pyrDown for 8UC1

**Description**
- used fma for sum calculating
- loading data using uchar4 for 8UC1

**Performance report**
http://ocl.itseez.com/intel/export/perf/pr/2882/report/

check_regression=*OCL*yrDown*
test_filter=*OCL*yrDown*
build_examples=OFF
test_modules=imgproc
are you sure that `srcData + mad24(_y, src_step, PIXSIZE * _x))` is properly aligned address?
OpenCL specifications says (see http://www.khronos.org/registry/cl/sdk/1.2/docs/man/xhtml/dataTypes.html):
>
A data item declared to be a data type in memory is always aligned to the size of the data type in bytes. For example, a float4 variable will be aligned to a 16-byte boundary, and a char2 variable will be aligned to a 2-byte boundary.