drm/radeon/kms: fix cursor image off-by-one error

The mouse cursor hotspot calculation when the cursor is partially off the
top or left side of the screen was off by one.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=41158



Signed-off-by: Nicholas Miell <nmiell@gmail.com>
Reviewed-by: Michel Dänzer <michel@daenzer.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>