drm/amd/display: Fix underflow when playing 8K video in full screen mode

[Why&How]
Flickering observed while playing 8k HEVC-10 bit video in full screen
mode with black border. We didn't support this case for subvp.
Make change to the existing check to disable subvp for this corner case.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Leo Ma <hanghong.ma@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>