drm/i915: Streamline skl_commit_modeset_enables()

skl_commit_modeset_enables() is a bit of mess. Let's streamline
it by simply tracking which pipes still need to be updated.
As a bonus we get rid of the state->wm_results.dirty_pipes usage.

v2: Rebase due to port sync

Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210144105.3239-2-ville.syrjala@linux.intel.com