video_core: Adjust `NUM` macro to avoid Clang warning

The previous definition was:

    #define NUM(field_name) (sizeof(Maxwell3D::Regs::field_name) / sizeof(u32))

In cases where `field_name` happens to refer to an array, Clang thinks
`sizeof(an array value) / sizeof(a type)` is an instance of the idiom
where `sizeof` is used to compute an array length.  So it thinks the
type in the denominator ought to be the array element type, and warns if
it isn't, assuming this is a mistake.

In reality, `NUM` is not used to get array lengths at all, so there is no
mistake.  Silence the warning by applying Clang's suggested workaround
of parenthesizing the denominator.