drm/i915: Use intel_ddi_get_config() for MST

drm/i915: Use intel_ddi_get_config() for MST

Eliminate the partially duplicated DDI readout code from MST, and
instead just call intel_ddi_get_config(). As a nice bonus we get
more cross checking as intel_ddi_get_config() will populate
output_types based on the actual mode of the DDI port.

Additonally intel_ddi_get_config() must be changed to get the crtc
from the passed in crtc state rather than from the encoder->crtc link.
encoder->crtc really shouldn't be used anyway.

v2: Rebased on BXT MST latency_optim fix
    Make intel_ddi_clock_get() static

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171027193128.14483-7-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>