drm/gma500: Convert to use new SCU IPC API

Convert the GMA500 driver to use the new SCU IPC API. This allows us
to get rid of the duplicate PMC IPC implementation which is now covered
in SCU IPC driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>