drm/radeon: Try harder to avoid HW cursor ending on a multiple of 128 columns.

This could previously fail if either of the enabled displays was using a
horizontal resolution that is a multiple of 128, and only the leftmost column
of the cursor was (supposed to be) visible at the right edge of that display.

The solution is to move the cursor one pixel to the left in that case.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=33183



Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>