drm: Purge ioctl forward declarations from drmP.h

If we push down the ioctl table in drm_ioctl.c all the forward
declarations in drmP.h are not required any more.

v2: Fold in fixup from Fenugguang Wu to declare functions as static.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>