Enable DSC using the DSC AUX device stored for this purpose in the
connector. For clarity add separate functions to enable/disable the
decompression, since these sequences will diverge more in follow-up
patches that also enable/disable DSC passthrough and on MST do the
actual enabling/disabling only for the first/last user of the given
AUX device. As a preparation for the latter refcounting change, also
pass the atomic state to the functions.

While at it set/clear only the DP_DECOMPRESSION_EN flag in the
DP_DSC_ENABLE DPCD register, preserving the reserved register bits.

Besides preserving the reserved register bits, the behavior stays as
before, as DSC is still only enabled for the first MST stream (which a
follow-up patch changes, enabling it for all streams).

v2:
- Add a helper function setting/clearing the decompression flag,
  preserving the reserved register bits.
v3:
- Add separate functions to enable/disable decompression and pass the
  atomic state to these.
- Add DocBook for both functions.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v2)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-24-imre.deak@intel.com