video-uclass: Fix logical-not-parentheses warning

With clang-4.0 and later we see:
warning: logical not is only applied to the left hand side of this bitwise
operator [-Wlogical-not-parentheses]
        if ((!gd->flags & GD_FLG_RELOC))
             ^          ~

And while the compiler suggests adding parenthesis around gd->flags, a
reading of the code says that we want to know when GD_FLG_RELOC is not
set and then return.

Cc: Simon Glass <sjg@chromium.org>
Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>