drm/dp_mst: Add DSC enablement helpers to DRM

Adding a helper function to be called by
drivers outside of DRM to enable DSC on
the MST ports.

Function is called to recalculate VCPI allocation
if DSC is enabled and raise the DSC flag to enable.
In case of disabling DSC the flag is set to false
and recalculation of VCPI slots is expected to be done
in encoder's atomic_check.

v2: squash separate functions into one and call it per
port
v3: Fix comment typos

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>