gma500/cdv: Fix call to cdv_intel_dp_set_m_n

We should be making this call not praying that the values are right.
In addition as noted by Josiah Standing we should be calling this
for eDP as well.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>