drm/amd/display: Assign preferred stream encoder instance to dpia

drm/amd/display: Assign preferred stream encoder instance to dpia

[Why]
For dpia, preferred engine instance availability is not checked
when assigning stream encoder instance.

[How]
Check for dpia preferred engine id and assign the same stream
encoder instance for the stream if available.

Reviewed-by: PeiChen Huang <peichen.huang@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>