OCL: Small fixes (test_loop_times = 30)

Fixed errors which found with test_loop_times = 30:
- pyrDown
- warpAffine (INTER_NEAREST)
- remap (INTER_LINEAR)
- calcHist

test_modules=imgproc
build_examples=OFF
check_regression=OCL*PyrDown*:OCL*WarpAffine*:OCL*Remap*:OCL*CalcHist*
test_filter=OCL*PyrDown*:OCL*WarpAffine*:OCL*Remap*:OCL*CalcHist*
would not be better `src += src_offset` before the loop?