drm/i915: replace is_cpu_edp() with a check for port A

The patch changes all remaining is_cpu_edp() check with a check for port
A. We can do this, since in all these cases ValleyView is handled
separately and port A is always a CPU side eDP port.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>