drm/qxl: use drm_crtc_vblank_{get,put}()

Replace the legacy drm_vblank_{get,put}() with the new helper functions.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465224105-21485-12-git-send-email-gustavo@padovan.org