drm/amd/display: reuse current context instead of recreating one

[Why]
Currently we discard the current context and recreate it. The current
context is what is applied to the HW so we should be re-using this
rather than creating a new context.

Recreating the context can lead to mismatch between new context and the
current context

For example: gsl groups get changed when we create a new context this
can cause issues in a multi display config (with flip immediate) because
we don't align the existing gsl groups in the new and current context.
If we reuse the current context the gsl group assignment stays the same.

[How]
Instead of discarding the current context, we instead just copy the
current state and add/remove planes and streams.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>