drm/i915: Don't flag both full modeset and fastset at the same time

Be consistent in whether we flag a full modeset or a
fastset for the pipe. intel_modeset_all_pipes() would
seem to be the only codepath not getting this right.

The other case is when we flag the fastset initially,
currently we just clear the mode_changed flag and set
the update_pipe flag. But we could still have
connectors_changed==true or active_changed==true forcing
a full modeset anyway. So check for that after clearing
the mode_changed flag.

And let's add a WARN to make sure we did get it right.

v2: Deal with {connectors,active}_changed

Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221021162442.27283-4-ville.syrjala@linux.intel.com