drm/amd/display: Disable SubVP if test pattern is enabled

[Why]
Enabling DPG causes HUBP to stay in blank constantly. If DPG is enabled
while an MCLK switch is taking place with SubVP, it will cause the MCLK
to never complete. This is because SubVP MCLK switch relies a HUBP
VLine interrupt, which will never occur when HUBP is constantly in
blank.

[How]
Disable SubVP when test pattern is enabled.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>