drm/amd/display: Fixed read/write pointer issue for get dmub trace

[Why]
Driver get wrap around dmub trace data due to read pointer being
increased incorrectly when there are multiple interrupt
queues with very short interval

[How]
Check read/write pointer before copying data from ring buffer

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>