drm/radeon/kms: only change mode when coherent value changes.

On X startup we were getting a flicker where there shouldn't have been one.
the X DDX calls the kernel to set the properties to the same values (yes
it could be smarter), however the kernel was doing a pointless modeset then,
making my nice smooth boot ugly.

Signed-off-by: Dave Airlie <airlied@redhat.com>