drm/radeon/dp: check for errors in dpcd reads

Check to make sure the transaction succeeded before
using the register value.  Fixes occasional link training
problems.

Noticed-by: Sergei Antonov <saproj@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>