radeon/kms: fix dp displayport mode validation

Check if there is a big enough dp clock & enough dp lane to
drive the video mode provided.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-By: Alex Deucher <alexdeucher@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Dave Airlie <airlied@gmail.com>