drm/i915: Remove unnecessary PORT3 definition.

Let's be picky and just use PICK directly.
So we can extend this later without creating
a new PORT_X por every new number of ports we
have to handle.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496700722-13755-1-git-send-email-rodrigo.vivi@intel.com