drm/radeon/kms: fix calculation of h/v scaling factors

Prior to this patch the code was dividing the src_v by the dst_h
and vice versa, rather than src_v/dst_v and src_h/dst_h.

This could lead to problems in the calculation of the display
watermarks.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>