video: grvga: Use module_platform_driver

module_platform_driver removes some boilerplate code and makes
it simple.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Kristoffer Glembo <kristoffer@gaisler.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>