drm/amd/display: Handle pixel format test request

[Why]
Some DSC tests fail because stream pixel encoding does not change
its value according to the type requested in the DPCD test params.

[How]
Set stream pixel encoding before updating DSC config and configuring
the test pattern.

Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Reviewed-by: Hanghong Ma <Hanghong.Ma@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>