drm/i915: Encapsulate dbuf state handling harder

drm/i915: Encapsulate dbuf state handling harder

In order to make the dbuf state computation less fragile
let's make it stand on its own feet by not requiring someone
to peek into a crystall ball ahead of time to figure out
which pipes need to be added to the state under which potential
future conditions. Instead we compute each piece of the state
as we go along, and if any fallout occurs that affects more than
the current set of pipes we add the affected pipes to the state
naturally.

That requires that we track a few extra thigns in the global
dbuf state: dbuf slices for each pipe, and the weight each
pipe has when distributing the same set of slice(s) between
multiple pipes. Easy enough.

We do need to follow a somewhat careful sequence of computations
though as there are several steps involved in cooking up the dbuf
state. Thoguh we could avoid some of that by computing more things
on demand instead of relying on earlier step of the algorithm to
have filled it out. I think the end result is still reasonable
as the entire sequence is pretty much consolidated into a single
function instead of being spread around all over.

The rough sequence is this:
1. calculate active_pipes
2. calculate dbuf slices for every pipe
3. calculate total enabled slices
4. calculate new dbuf weights for any crtc in the state
5. calculate new ddb entry for every pipe based on the sets of
   slices and weights, and add any affected crtc to the state
6. calculate new plane ddb entries for all crtcs in the state,
   and add any affected plane to the state so that we'll perform
   the requisite hw reprogramming

And as a nice bonus we get to throw dev_priv->wm.distrust_bios_wm
out the window.

v2: Keep crtc_state->wm.skl.ddb

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122205633.18492-8-ville.syrjala@linux.intel.com