drm/amd/display: remove context->dml2 dependency from DML21 wrapper

[Why]
When the DML2 wrapper explicitly accesses context->dml2, that creates a
dependency on where dc saves the DML object. This dependency makes it
harder to have multiple co-existing DML objects, which we would like to
have for upcoming functionality.

[How]
 - make all DML21 interfaces take in a DML2 object as parameter
 - remove all references to context->dml2, use parameter instead

Reviewed-by: Jun Lei <jun.lei@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>