drm: allow removal of legacy codepaths (v4.1)

If you don't want the legacy drivers, then lets get rid of all the
legacy codepaths from the core module.

This drop the size of drm.ko for me by about 10%.
 380515    7422    4192  392129   5fbc1 ../../drm-next-build/drivers/gpu/drm/drm.ko
 351736	   7298	   4192	 363226	  58ada	../../drm-next-build/drivers/gpu/drm/drm.ko

v2: drop drm_lock as well, fix some DMA->DRM typos
v3: avoid ifdefs in mainline code
v4: rework ioctl defs
v4.1: fix nouveau Kconfig

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>