drm/dp_mst: Fix fractional DSC bpp handling

The current code does '(bpp << 4) / 16' in the MST PBN
calculation, but that is just the same as 'bpp' so the
DSC codepath achieves absolutely nothing. Fix it up so that
the fractional part of the bpp value is actually used instead
of truncated away. 64*1006 has enough zero lsbs that we can
just shift that down in the dividend and thus still manage
to stick to a 32bit divisor.

And while touching this, let's just make the whole thing more
straightforward by making the passed in bpp value .4 binary
fixed point always, instead of having to pass in different
things based on whether DSC is enabled or not.

v2:
- Fix DSC kunit test cases.

Cc: Manasi Navare <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Harry Wentland <[email protected]>
Cc: David Francis <[email protected]>
Cc: Mikita Lipski <[email protected]>
Cc: Alex Deucher <[email protected]>
Fixes: dc48529fb14e ("drm/dp_mst: Add PBN calculation for DSC modes")
Signed-off-by: Ville Syrjälä <[email protected]>
[Imre: Fix kunit test cases]
Acked-by: Maxime Ripard <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Acked-by: Harry Wentland <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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