drm/i915: handle DP_MST correctly in bxt_get_dpll

No idea if it supports it, but this is the minimum required from get_dpll.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470755054-32699-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>