OCL: added extra cvtColor modes

Added OCL kernels for following conversions:
- `YUV_NV21` -> `BGR[A]` | `RGB[A]`
- `YUV_YV12` | `YUV_IYUV` -> `BGR[A]` | `RGB[A]`
- `YUV_420` -> `GRAY`
- `BGR[A]` | `RGB[A]` -> `YUV_YV12` | `YUV_IYUV`
- `YUV_UYVY` | `YUV_YUY2` | `YUV_YVYU` -> `BGR[A]` | `RGB[A]`

**Performance report**
http://ocl.itseez.com/intel/export/perf/pr/3318/report/

check_regression=*OCL*Color*
test_filter=*OCL*Color*
test_modules=imgproc
build_examples=OFF
Yes, but I need some time to prepare version with optimal accuracy/performance ratio. I tried to make these calculations directly in floats and get very good accuracy. I decided to apply this scheme wherever is possible.