splash: fix build breakage with newer llvm-7

Some configurations (i.e. sandbox) result in unused
splash_display_banner() and we get build errors like:

  common/splash.c:148:20: error: unused function 'splash_display_banner' [-Werror,-Wunused-function]
  static inline void splash_display_banner(void) { }

Drop empty inline function and add guards around the call.

Fixes: d2a8271c ("splash: fix splash banner output")
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reported-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>