drm/msm/dpu: enumerate second cursor pipe for external interface

Setup an RGB HW pipe as cursor which can be used on
secondary interface.

For SC7180 2 HW pipes are enumerated as cursors
1 - primary interface
2 - secondary interface

Signed-off-by: Kalyan Thota <kalyan_t@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>