"If I have some time I'll come by tomorrow." - this is what you need. This may well happen. "If I had some time I'd come by tomorrow." - this tells me the intention is there, but the chances of it happening are unlikely. Note how "I will" changed to "I would".

"If I have..." is correct: this is a sentence in the first conditional form. "Had" is reserved for the past tense, or for a second conditional sentence ("If I had some time I would go to his house") or a third conditional sentence ("If I had had some time, I would have gone to his house").