drm/nouveau: Only recalculate PBN/VCPI on mode/connector changes

I -thought- I had fixed this entirely, but it looks like that I didn't
test this thoroughly enough as we apparently still make one big mistake
with nv50_msto_atomic_check() - we don't handle the following scenario:

* CRTC #1 has n VCPI allocated to it, is attached to connector DP-4
  which is attached to encoder #1. enabled=y active=n
* CRTC #1 is changed from DP-4 to DP-5, causing:
  * DP-4 crtc=#1→NULL (VCPI n→0)
  * DP-5 crtc=NULL→#1
  * CRTC #1 steals encoder #1 back from DP-4 and gives it to DP-5
  * CRTC #1 maintains the same mode as before, just with a different
    connector
* mode_changed=n connectors_changed=y
  (we _SHOULD_ do VCPI 0→n here, but don't)

Once the above scenario is repeated once, we'll attempt freeing VCPI
from the connector that we didn't allocate due to the connectors
changing, but the mode staying the same. Sigh.

Since nv50_msto_atomic_check() has broken a few times now, let's rethink
things a bit to be more careful: limit both VCPI/PBN allocations to
mode_changed || connectors_changed, since neither VCPI or PBN should
ever need to change outside of routing and mode changes.

Changes since v1:
* Fix accidental reversal of clock and bpp arguments in
  drm_dp_calc_pbn_mode() - William Lewis

Signed-off-by: Lyude Paul <[email protected]>
Reported-by: Bohdan Milar <[email protected]>
Tested-by: Bohdan Milar <[email protected]>
Fixes: 232c9eec417a ("drm/nouveau: Use atomic VCPI helpers for MST")
References: 412e85b60531 ("drm/nouveau: Only release VCPI slots on mode changes")
Cc: Lyude Paul <[email protected]>
Cc: Ben Skeggs <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Jerry Zuo <[email protected]>
Cc: Harry Wentland <[email protected]>
Cc: Juston Li <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Karol Herbst <[email protected]>
Cc: Ilia Mirkin <[email protected]>
Cc: <[email protected]> # v5.1+
Acked-by: Ben Skeggs <[email protected]>
Signed-off-by: Dave Airlie <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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