drm/amd/display: fix null pointer deref when plugging in display

[Why&How]
When system boots in headless mode, connecting a 4k display creates a
null pointer dereference due to hubp for a certain plane being null.
Add a condition to check for null hubp before dereferencing it.
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn30/dcn30_hwseq.c

@@ -969,6 +969,7 @@ void dcn30_set_disp_pattern_generator(const struct dc *dc,
 		/* turning off DPG */
 		pipe_ctx->plane_res.hubp->funcs->set_blank(pipe_ctx->plane_res.hubp, false);
 		for (mpcc_pipe = pipe_ctx->bottom_pipe; mpcc_pipe; mpcc_pipe = mpcc_pipe->bottom_pipe)
+			if (mpcc_pipe->plane_res.hubp)
 				mpcc_pipe->plane_res.hubp->funcs->set_blank(mpcc_pipe->plane_res.hubp, false);
 
 		stream_res->opp->funcs->opp_set_disp_pattern_generator(stream_res->opp, test_pattern, color_space,