drm/amd/display: reverted limiting vscsdp_for_colorimetry and ARGB16161616 pixel format addition

[WHY]
Limiting vscsdp_for_colorimetry for YCbCr420/BT2020 resulted in red/green
point failures in HDR10 DTN tests. The re-implementation of ARGB16161616
was to fix this however it did not actually fix this issue but a side effect of the
issue.

[HOW]
Change ARGB16161616 pixel format to 26.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Ethan Wellenreiter <Ethan.Wellenreiter@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>