drm/i915: Consolidate DDI clock reading out in a single function

2 pieces of code need to read out the DDI clock: the DDI encoder and the
MST encoder .get_config() vfuncs.

Until now the SKL read out code was only in the former, so let's move
the pre and post SKL logic in intel_ddi_clock_get() and this this one
everywhere.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>