drm/amd/display: don't clean-up bottom pipe plane_state

In the scenario of setting underlay, dc_validate_global_state()
is called after constructing the context with all
relevant bottom_pipe related configurations in dm_update_planes_state().

Currently, in dc_validate_global_state(), without checking for bottom_pipe's
existence, the pipe_ctx structure is initialised to 0, hence
nullyfying the plane_state of bottom_pipe which shall be accessed
in populate_initial_data() called from bw_calcs().

Due to this null pointer access kernel panics and leads to reboot
when underlay is tried to set.

This patch fixes the issue by no longer clearing the top_pipe. This
workaround is no longer required.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>