drm: qxl: Replace CRTC .commit() helper operation with .enable()

The CRTC helper .commit() operation is legacy code, the atomic helpers
prefer the .enable() operation. Replace the .commit() helper operation
with .enable() in the driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170627211621.27767-4-laurent.pinchart+renesas@ideasonboard.com