drm/amd/display: Increase DP blank timeout from 30 ms to 50 ms

[Why]
At 24 Hz, a frame is 41.7 ms, so a 30 ms wait can (and does often)
timeout.

[How]
Bump timeout from 30 ms to 50 ms.

Signed-off-by: Ken Chalmers <ken.chalmers@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>