drm/mode: Unstatic kernel-userspace mode conversion

Move the drm_display_mode <-> drm_mode_modeinfo conversion functions
from drm_crtc.c to drm_modes.c, and make them non-static so that others
can use them.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>