matroxfb: perform a dummy read of M_STATUS

commit 972754cfaee94d6e25acf94a497bc0a864d91b7e upstream.

I had occasional screen corruption with the matrox framebuffer driver and
I found out that the reason for the corruption is that the hardware
blitter accesses the videoram while it is being written to.

The matrox driver has a macro WaitTillIdle() that should wait until the
blitter is idle, but it sometimes doesn't work. I added a dummy read
mga_inl(M_STATUS) to WaitTillIdle() to fix the problem. The dummy read
will flush the write buffer in the PCI chipset, and the next read of
M_STATUS will return the hardware status.

Since applying this patch, I had no screen corruption at all.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>