drm/radeon: Fix rs400_gpu_init for ATI mobility radeon Xpress 200M

num_gb_pipes was set to a wrong value using r420_pipe_config

This have lead to HyperZ glitches on fast Z clearing.

Closes: https://bugs.freedesktop.org/show_bug.cgi?id=110897


Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Richard Thier <u9vata@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 044e59a8)
Cc: stable@vger.kernel.org