drm/amd/display: Set test pattern on blank when using Visual Confirm

[Why]
We want a test pattern to show up on screen when we're blanked and have
visual confirm enabled, for debugging. Raven does this, it's a mistake that
Navi does not.

[How]
 - in "blank_pixel_data", set appropriate DPG pattern for visual confirm
 - refactor DPG calls out of "enable_stream_timing"

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>