DRM: UDL: get rid of useless vblank initialization

UDL doesn't support vblank functionality so we don't need to
initialize vblank here (we are able to send page flip
completion events even without vblank initialization)

Moreover current drm_vblank_init call with num_crtcs > 0 causes
sending DRM_EVENT_FLIP_COMPLETE event with zero timestamp every
time. This breaks userspace apps (for example weston) which
relies on timestamp value.

Cc: stable@vger.kernel.org
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928144126.21598-1-Eugeniy.Paltsev@synopsys.com