drm: Remove drm_modeset_(un)lock_crtc

The last user, the cursor ioctl, can just open-code this too. We
simply have to move the acquire ctx dance from the universal function
up into the top-level ioctl handler.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-3-daniel.vetter@ffwll.ch