drm/msm/dpu: enable cursor plane on dpu

Reserve DMA pipe for cursor plane and attach it to the
crtc during the initialization.

Changes in V2:
	None

Signed-off-by: Sravanthi Kollukuduru <skolluku@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>