drm/i915: Rework global state locking

drm/i915: Rework global state locking

So far we've sort of protected the global state under dev_priv with
the connection_mutex. I wan to change that so that we can change the
cdclk even for pure plane updates. To that end let's formalize the
protection of the global state to follow what I started with the cdclk
code already (though not entirely properly) such that any crtc mutex
will suffice as a read lock, and all crtcs mutexes act as the write
lock.

We'll also pimp intel_atomic_state_clear() to clear the entire global
state, so that we don't accidentally leak stale information between
the locking retries.

As a slight optimization we'll only lock the crtc mutexes to protect
the global state, however if and when we actually have to poke the
hw (eg. if the actual cdclk changes) we must serialize commits
across all crtcs so that a parallel nonblocking commit can't get
ahead of the cdclk reprogamming. We do that by adding all crtcs to
the state.

TODO: the old global state examined during commit may still
be a problem since it always looks at the _latest_ swapped state
in dev_priv. Need to add proper old/new state for that too I think.

v2: Remeber to serialize the commits if necessary

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015193035.25982-3-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>