drm/amd/display: Add null check for head_pipe in dcn32_acquire_idle_pipe_for_head_pipe_in_layer

This commit addresses a potential null pointer dereference issue in the
`dcn32_acquire_idle_pipe_for_head_pipe_in_layer` function. The issue
could occur when `head_pipe` is null.

The fix adds a check to ensure `head_pipe` is not null before asserting
it. If `head_pipe` is null, the function returns NULL to prevent a
potential null pointer dereference.

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/../display/dc/resource/dcn32/dcn32_resource.c:2690 dcn32_acquire_idle_pipe_for_head_pipe_in_layer() error: we previously assumed 'head_pipe' could be null (see line 2681)

Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/resource/dcn32/dcn32_resource.c

@@ -2678,8 +2678,10 @@ static struct pipe_ctx *dcn32_acquire_idle_pipe_for_head_pipe_in_layer(
 	struct resource_context *old_ctx = &stream->ctx->dc->current_state->res_ctx;
 	int head_index;
 
-	if (!head_pipe)
+	if (!head_pipe) {
 		ASSERT(0);
+		return NULL;
+	}
 
 	/*
 	 * Modified from dcn20_acquire_idle_pipe_for_layer