drm/amd/display: return 'NULL' instead of 'false' from dcn20_acquire_idle_pipe_for_layer

clang complains that 'false' is a not a pointer:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_resource.c:2428:10: error: expression which evaluates to zero treated as a null pointer constant of type 'struct pipe_ctx *' [-Werror,-Wnon-literal-null-conversion]
                return false;

Changing it to 'NULL' looks like the right thing that will shut up
the warning and make it easier to read, while not changing behavior.

Fixes: 7ed4e635 ("drm/amd/display: Add DCN2 HW Sequencer and Resource")
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c

@@ -2415,7 +2415,7 @@ struct pipe_ctx *dcn20_acquire_idle_pipe_for_layer(
 		ASSERT(0);
 
 	if (!idle_pipe)
-		return false;
+		return NULL;
 
 	idle_pipe->stream = head_pipe->stream;
 	idle_pipe->stream_res.tg = head_pipe->stream_res.tg;