hw/intc/arm_gic: modernise the DPRINTF

While I was debugging the icount issues I realised a bunch of the
messages look quite similar. I've fixed this by including __func__ in
the debug print. At the same time I move the a modern if (GATE) style
printf which ensures the compiler can check for format string errors
even if the code gets optimised away in the non-DEBUG_GIC case.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
