obs-nvenc: Force at least 4 b-frames when using UHQ tune

[derrod]

Description

Forces the number of b-frames to 4 if it is below 4.

Motivation and Context

Users running into non-descriptive errors when trying to use UHQ tune without setting the correct number of b-frames.

How Has This Been Tested?

Hasn't.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.