drm: Move drm_getmap into drm_bufs.c and give it a legacy prefix

It belongs right next to the addmap and rmmap functions really. And
for OCD consistency name it drm_legacy_getmap_ioctl.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-4-git-send-email-daniel.vetter@ffwll.ch