drm/msm: dpu: Don't bother checking ->enabled in dpu_crtc_vblank

The drm_crtc_vblank_on/off calls in enable/disable guarantee that we
won't call this function when crtc is not enabled.

Changes in v2:
- None

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>

Signed-off-by: Rob Clark <robdclark@gmail.com>