Move common code from common_xxx to common

[andr2000]

Make platform driver implementation simpler by moving common code from sample platform driver into common.c

[aanisov]

As I remember it was an intention to do so once we have something functional and see what else is generic.

[andr2000]

The code already has candidates to be moved out to the common part: just like DRM does:
drm_dev_alloc
drm_dev_register
drm_dev_unregister