Don't cast -1 to enum to represent a missing value.

This is undefined behavior in C++, and a clang warning suggests it is
actually producing bad code as a result:

../Source/Core/VideoCommon/BPFunctions.cpp:164:45: warning: comparison of constant 4294967295 with expression of type 'PEControl::PixelFormat' is always false [-Wtautological-constant-out-of-range-compare]
        if (new_format == old_format || old_format == (unsigned int)-1)
Fix harmless warnings.