The MDP5 plane's atomic_check ops doesn't perform clipping tests.
This didn't hurt us much in the past, but clipping becomes important
with cursor planes.

Use drm_plane_helper_check_state, the way rockchip/intel/mtk drivers
already do. Use these drivers as reference.

Clipping requires knowledge of the crtc width and height. This requires
us to call drm_atomic_helper_check_modeset before
drm_atomic_helper_check_planes in the driver's atomic_check op, because
check_modetest will populate the mode for the crtc, needed to populate
the clip rectangle.

We update the plane_enabled(state) local helper to use state->visible,
since state->visible and 'state->fb && state->crtc' represent the same
thing.

One issue with the existing code is that we don't have a way to disable
the plane when it's completely clipped out. Until there isn't an update
on the crtc (which would de-stage the plane), we would still see the
plane in its last 'visible' configuration.
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>