drm/amd/display: Backup and restore only on full updates

drm/amd/display: Backup and restore only on full updates

[WHY & HOW]
Since the backup and restore for plane and stream states has
a significant amount of data to copy, we will change the backup
and restore sequence to only take place during full updates.

We will also move the scratch memory to struct dc instead of dc_state
to avoid needing to allocate large amounts of memory every time
we create a new DC state.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>