drm/msm/dpu: move pstate->pipe initialization to dpu_plane_atomic_check

In preparation for virtualized planes support, move pstate->pipe
initialization from dpu_plane_reset() to dpu_plane_atomic_check(). In
case of virtual planes the plane's pipe will not be known up to the
point of atomic_check() callback.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/621477/
Link: https://lore.kernel.org/r/20241025-dpu-virtual-wide-v6-2-0310fd519765@linaro.org