drm/i915/bxt: Add MST support when do DPLL calculation

Add the missing INTEL_OUTPUT_DP_MST case in bxt_get_dpll()
to correctly initialize the crtc_state and port plls when
link training a DP MST monitor on BXT/APL devices.

Fixes: a277ca7dc01d ("drm/i915: Split bxt_ddi_pll_select()")
Bugs: https://bugs.freedesktop.org/show_bug.cgi?id=99572
Reviewed-by: Cooper Chiou <cooper.chiou@intel.com>
Reviewed-by: Gary C Wang <gary.c.wang@intel.com>
Reviewed-by: Ciobanu, Nathan D <nathan.d.ciobanu@intel.com>
Reviewed-by: Herbert, Marc <marc.herbert@intel.com>
Reviewed-by: Bride, Jim <jim.bride@intel.com>
Reviewed-by: Navare, Manasi D <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: <stable@vger.kernel.org> # v4.9+
Signed-off-by: Lee, Shawn C <shawn.c.lee@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486096329-6255-1-git-send-email-shawn.c.lee@intel.com
(cherry picked from commit 0aab2c721d81590012a5021a516f00666646741f)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>