drm/i915/skl: Explicitly check for eDP in skl_ddi_pll_select()

Assuming any connector that isn't DP, MST, or HDMI is eDP definitely
seems likely to cover up other bugs in the future.

Signed-off-by: Lyude <cpaul@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1454423709-21882-2-git-send-email-cpaul@redhat.com